As a beginner, don't worry about tiers, especially if you're not even sure you'll be totally getting into the game. Seriously.
Tiers do exist, absolutely, but they really only mean something when you're playing at a higher level of play. Your first goal should be A.) picking characters you actually like and B.) picking characters that function well as a team. You'll pretty quickly find that having a team with good synergy is just as, if not more, important than having characters that are "strong" individually.
That said, if there is one character I would avoid playing as a newcomer to game, it's Phoenix/Jean Gray. She can be a very powerful character in the right hands, but because of how she works she forces your whole team to play differently than it would otherwise. If you're not aware, if she dies while you have 5 super bars stocked up then she turns into Dark Phoenix; a super powered version of herself that has been weakened in UMvC3 but can still fairly easily destroy pretty much anyone. The problem with this is that you start playing to build 5 bars and then not use them, which is pretty much the opposite of what you do with any other character/team. Also, because she can be so powerful, using her can become a crutch that hurts how well you learn to play the rest of your team. Even at the pro level, there are (or were in vanilla at least) a decent number of Phoenix players out there but very few of even the pros could win matches without having to consistently rely on Dark Phoenix comebacks.
Anyway, if you're looking to learn I would heartily suggest going to shoryuken.com. They have a great MvC3 guide there, extremely deep character forums, and multiple daily links to videos of tutorials and new stuff that's being discovered. Just don't bother actually trying to participate in their comments/forums as many of the regular users tend to the elitist jackass end of things.
In short, pick who you like instead of who's "strong," spend lots of time in training mode, and go to shoryuken.com once you've settled in a bit and want to dig into real information.
Tiers do exist, absolutely, but they really only mean something when you're playing at a higher level of play. Your first goal should be A.) picking characters you actually like and B.) picking characters that function well as a team. You'll pretty quickly find that having a team with good synergy is just as, if not more, important than having characters that are "strong" individually.
That said, if there is one character I would avoid playing as a newcomer to game, it's Phoenix/Jean Gray. She can be a very powerful character in the right hands, but because of how she works she forces your whole team to play differently than it would otherwise. If you're not aware, if she dies while you have 5 super bars stocked up then she turns into Dark Phoenix; a super powered version of herself that has been weakened in UMvC3 but can still fairly easily destroy pretty much anyone. The problem with this is that you start playing to build 5 bars and then not use them, which is pretty much the opposite of what you do with any other character/team. Also, because she can be so powerful, using her can become a crutch that hurts how well you learn to play the rest of your team. Even at the pro level, there are (or were in vanilla at least) a decent number of Phoenix players out there but very few of even the pros could win matches without having to consistently rely on Dark Phoenix comebacks.
Anyway, if you're looking to learn I would heartily suggest going to shoryuken.com. They have a great MvC3 guide there, extremely deep character forums, and multiple daily links to videos of tutorials and new stuff that's being discovered. Just don't bother actually trying to participate in their comments/forums as many of the regular users tend to the elitist jackass end of things.
In short, pick who you like instead of who's "strong," spend lots of time in training mode, and go to shoryuken.com once you've settled in a bit and want to dig into real information.