To add to all the other advice here:
Your best bet at a 1-star team is Iron Man Model 35, Modern Black Widow, Modern Storm. Very well-rounded team that uses all colors.
Your best bet at a 2-star team is Thor, Classic Storm, Whoever (Astonishing X-Men Wolverine is fine, especially if you focus on developing his green ability and let Thor use your red AP for damage). All 3 of these characters are available as rewards in prologue missions. I agree with the suggestion of trying to collect 2-star covers ASAP, particularly Thor and Classic Storm, since they are the best 2-star characters in the game and still very viable in PVP.
Consider friending lots of people on Facebook who play this game. Or, if you don’t want to use your personal FB account, create a separate FB account just for MPQ friends (but be warned that FB may think your new account is fake, especially if you rapidly add a ton of new people within 1-2 days). Attach whichever FB account you decide to use to your game so that you can share, and be shared, extra ISO and other rewards…it will help your leveling progress a lot.
Keep in mind that leveling your characters comes from spending ISO, not from repeated play sessions. i.e. If you play a match, your guys don’t “earn” any XP. Instead, you will win a small amount of ISO you can spend to level up your guys.
Once you crack open the Venom mission in the prologue, you will probably find yourself coming back to it repeatedly to have Classic Spider-Man (which the game gives you for free in certain battles) heal your other team members to full. Aside from Classic Spider-Man, very few other characters have the ability to heal themselves (the Wolverine variants have some kind of self-regeneration, and the new Original Black Widow also has a team heal). Aside from those abilities, there is currently no other way to heal your guys other than waiting for their health to slowly regenerate in real time, or using heal items, which replenish every 30 minutes or so. If you’re competing in a long-term event such as a multi-day tourney, it may make sense for you to use your heal items regularly because of the way heal replenishes work…once you are back up to 5 heals, you will not replenish any more, so for those long-term events of 24 hours or more, it may make sense to use them regularly so that you can replenish more later.