Lots of vague questions. To help you better next time I would try and be more specific.

Ok well Ill try and help you as best I can.
Lets start with the basics.
Mewtwo is not going to be a fun main. Yes I said it not fun, if you don't liek to work your butt off for every stock and generally accept that you will always be at a greater disadvantage to everyone you play. Your going to loose alot. So get used to it. Don't get mad tho. You just need to keep at it.
Ok now the advanced tecks.
M2 has the third best WD in the game. You need to capitalize on it. You NEED to have a PERFECT WD if you want to get good with m2. No exeptions. Your WD need's to be far reaching and you need to be able to do them on the fly, with no hisitation. Because you are just learning the WD your probably not going have it down pat just yet, however I would definatly practice your WD daily. WD is how M2 moves around the board. If you want to keep up with the faster characters ie. Fox, sheik ext. Your going to need to get good at it.
Now that you get the WD your going to need to learn how to L-cancle. This teck is also extreamly important because it speeds up your game tremendusly. So I would also try practicing this on a daily basis.
Ok now onto a quick m2 guide.
M2 is a defensive character and really doesn't have any 'solid approaches'
Your approaches are as follows.
WD D-tilt
WD F-tilit
WD Grab******Extreamly usefull******
Shuffled nair (however this is definatly situational)
As you can see your definatly limited in your approaches. that is why M2 is generally played as a defensive character. Because of this you definatly need to get good at shield grabbing. Grabs are the core of m2's game. Withought the grabs he is nothing. So you always need to try ad find a way to utilize his grabs. because that is how your going to ba able to rack up % and KO. When Using Grabs your probably only going to be using three, they are
Uthrow
Dthrow
Bthrow
Fthrow: is most useless. However sometimes can be used to great effect in teams (we will get to that later)
Uthrow: Up throw is a great combo starter on Fast fallers (Fox Falco ext) at low percents. This grab should always be used when trying to KO a lighter opponent. Ie. Samus dies on Final destination at 120% the geral rule of thumb is Back throw Fast Fallers for the Ko and U throw lighter opponents for the Ko.
Dthrow: This grab is your bread and butter. D-throw with D-tilts work great on many opponents and also this is how your probably going to be racking up some good % with teck chasing. teck chasing out of the D-throw takes practice however with time you will get the hang of it. To teck chase effeciantly your definatly going to need to be able to use WD with D-tilt and F-tilt and WD grab.
Bthrow: Back throw is a nice grab for getting someone off of the board and should be used to KO Fast fallers and heavier opponents Definatly a great KO grab.
F-throw: toatly and utterly pointless 99% of the time. You cannot combo out of this grab and it is also the weakest. it's only use in single player is if you are somehow facing a ledge (ie. The fin of great fox on corneria) You can Fthrow your opponent into the wall and then all of the baby shadowballs will hit them and explode causing alot of damage. howver be warned that if your opponent hit's the wall before the shadowballs they can teck. Fthrow is most usefull in team battles becasue you can 'suck' the enemy opponent into your malestorm of shadowballs and trap both of the opposing team members.
Ok now onto the more in depth oproaches.
Like I said earlier M2 doesn't really have any solid approches. However with a good Wd he defiatly can be a thret. You need to look at your spacing carefully becasue you can definatly travel quite a way with a good WD f-tilt or WD D-tilt. Sometimes the distance can suprize your opponent so use it to your advantage. With the d-tilts you can try and chain the into more D-tilts however I wouldn't suggest useing more than two, possibly three. If you use more than that your opponent can easily DI away from you. Insted what I normally do is Grab them or use F-tilt to get them off the the platform if the oppertunity presents itself. This is one of m2's better approaches.
WD Grab. Once again M2's game is all about grabbing. The better you can WD the more likely you will have a chance at getting in a grab from long distance. This is a great approach and need's to be mastered along with Shield Grabbing. WD Grab is a great way to sneak in when your opponent is sufferning from LAG. Howver don't spam this approach because it can be easily punished.
The next approach is going to be WD F-tilt. This doesn't really have any comboing chances if you hit with this becasue it will most likly send your opponent flying. This is a usefull way to get your enemy off of the stage.
Now for arials.
Fair
Nair
Uair
Bair
Fair: This is a great move. The only problem is it's range is really short. So don't try approaching with it. It's best used in conjunction with D-tilt. ie. Dtilt to DjC Fair. It's also a great way to rack up damage on fast fallers. ie. when they are at low % 0-36 you can uthrow to fair for a nice little combo. I would try and only use fair when your opponent is 'tumbling' use it in combos. However don't try and approach with this move. this move is going to take alot of practice to master. If the situation presents itself you can use fair if you jump out of your shield for a nice little suprize. however please note that this teck is situational at best.
Nair: Nair is M2's solid air move. However this move as well has some downsides. It can be used to approach with, however make sure that you are carfull when you decide to use this move. Because it has low range and priority. However if you can use it properly this air move is a really great tool for M2 for racking up some nice percents.
Uair: Is a good move for edge guarding. You can 'carry' your opponent of the edge with is move. Don't try approaching with is move becasue it has no range.
Bair: This move is definatly tricky at first becasue the hit box is so wierd. Bair is a great edge guard and it's going to take practice to use it effectivly. usaually your only going to be using Bair for an edge guard. Don't depend on this air move in the heat of battle. It will get you no where.
Overall m2 isn't really that strong in the air. When playing M2 your going to want to stay on the ground.
Ok now that you kind of know the basics I would now try the following:
1. Read the M2 guide
2. Watch M2 vids in the moive database
3. Practice.
Now for your friend. he is playing Marth and Samus. Two of M2's worst nightmares. Here's why.
Marth: He out ranges and out prioritizes you in everything. So your at a huge disadvantage. Fsmash is a killer. Watch out for that. Also your going to have to know Marths sword distance extreamly well. Your going to have to Wave dance just in and out of his range. However be warned that the tip of marths sword causes a huge amount of damage and knockback. If you make a mistake your going to pay for it. i don't know how good your friend is, however I will tell you this. never take to the air. marths Fair is too good. M2 has nothing to stop it. Stick to the ground. Don't try and go airbore against marth unless it's absolutly nessesary or if you have him in a combo. So your best bet is going to be wave shielding and your going to have to try and make Marth screw up his spacing. In order to help you more im going to need to know how good your friend is with marth. Marths are extreamly situational and stratagies for playing a good marth are totally differn't than playing a noob marth. More info would be greatly appreciated.
Samus: Yet another of M2's worst match ups. Samus is difficult for M2 becasue you are forced to approach her. And M2 doesn't do well when he has to go on the offensive. If your friend likes to missle spam you can try a few things.
1. you can destroy the missles with M2's Fair. this teck isn't going to win you the match. the only thing that this can possibly do is hopefully make the samus realize that by missle spamming she is not getting anywhere fast.
2. try and rush her if the missles are just too much. howver this isn't always the best of ideas because like I said before M2 doesn't like it when he has to rush.
Samus isn't a Fast faller so combos are not the easiest thing. You just need to try and get as much damage you can get in with every opening. Be carefull of those missiles. You need to try at all cost to make her rush you. Although this is easier said than done. Try and KO samus with Uthrow because Samus has a great recovery, so Bthrow is kind of pointless.
Once again I don't know how good your friends samus is, so I cannot really help you that much. It would be great to have some more information.
Well More information on your friends abilities would have allowed me to explathe matchups in greater detail. If you can give me something more to work with I could give you some more indepth strats.
All in All it sounds like you just need to get the hang of M2. You need to practice your WD, Lcancle Fast Falls DjC and what not. Try and just understand m2's movement and work ou your character control.
Hope this helps, also please feel free to PM me if you need more help. Also next time before posting please look around the boards you could have found all of your answers there
