Seriously?
You should get an asian friend or two, they will help you with that.
At least, that's how I think I got so good. SSBB and MvC3 were a large part of my senior year in highschool/freshman college so I think I am pretty good.
But REAL TALK:
H/Heavy = circle/B
M/Medium = triangle/Y
L/Light = square/X
S/Special = X/A
What teams are you trying?
Amateratsu is a good beginner character since: forwardH(ie, running towards them and hitting Circle/B) + H + H + H + H + H + S + jump + M + M + H + S is a very easy, like, stupidly easy, combo that does a shocking amount of damage. Or if you are more advanced or wanting a bigger punch and have 3 super meter ready, you can just forwardH combo and instead of using the Special, just use his level 3 hyper and it will auto hit them and deal significant damage if you can time it AS SOON AS THE LAST COMBO OF HEAVY ATTACKS ENDS. Another fun thing, his backHyper slows time, good for when you are playing against someone who is better at comboing than you since during slowed time, combos usually will be interrupted if they try to start one.
Hulk is also a great beginner character. His Heavy attack has super armor and thus can usually never be interrupted (even if he is hit mid swing) thus if you can connect an easy to his H you can H + S + jump + M + M + H + S + land on ground + GAMMA TSUNAMI for another shockingly large amount of dmg. If they are ranged and wont let you approach, airSpecial is an overhead AND it knocks them to the ground. So instead of starting with H, you can airSpecial + land + S + jump + M + M + H + S + land on ground + GAMMA TSUNAMI
Finally: Sentinal. **** yeah. This guy. MMmmmm. Heavy = laser attack, Duck heavy = low laser attack. If they aren't jumping, spam that ****. The trick is to alternate Heavy, downHeavy, Heavy, downHeavy (since if they block or it connects, it cancels and you can IMMEDIATELY followup, so spam that button if you see they aren't going to dodge it). They will not be able to move and will just take a crap ton of chip dmg. However, if they jump towards you, then you can usually jump + special and combo, or just Special and combo haha, but your lasers will only hurt you if they dodge it since the cooldown off it sucks if it doesnt hit/get blocked.
His downMedium also has super armor.
So if you can get in training and in a corner try downMedium + S + jump + M + M + H + S + land + RocketPunch(light) + Overload/ForwardHyper + RocketPunch(medium) + S + M + M + H + AirHyper (at least i think this is how it works, my roomie took PS3 for Spring break so I can't test to make sure...)
Note: Alot of combos in air usually consist of MMHS or LMMHS if you have a faster character.