I will not, no matter what you are thinking, give on this site a complete rundown of how your stats should match your concept and whatnot. If I really need to stress this that much to you, then leave this site immediatly. Honestly, you shouldn't even be a gamer if you're too dumb to know that RPGs ar ROLE-PLAYING GAMES.
But seriously, a character concept, especially in Mage, needs a lot of thought. You need to consider everything from gender to his/her political and religious beliefs. A group with well thought-out characters can pass downtime merely by talking, in character, of course, to each other, and they should have a fairly good grasp of their pasts, mentors, parents, etc. I have never seen this achieved, but it cannot be stressed enough. Characters know their pasts! They know their siblings, their parents, their friends! But if, as a player, you don't know these things, you're character won't either. An unprepared player can't even hold their own in a five-minute argument, because his character's opinions keep changing! Trust me, I've seen it happen.
All right, away from the happy crap. Get down into the three cardinal evils of gaming. They are: metagaming, rules lawyering, and munchkining. Metagaming is when someone, usually intentionally, uses out-of-game knowledge the aquire an in-game effect. For instace, if a character is homosexual, and a player who's playing a homophobe knows this, he may mock the first character about it in-game, even though there is no way the homophobe could hove found out that information. Rules Lawyering is when someone has no sense of 'dude, rules change from what the book says.' These players will stand and say stuff like, 'You can't do that, it says so on page....' These people are pains in the @$$. You cannot imagine how much damage they can cause to a game. The last of the Evils, the Munchkins, are those people who are completely obsessed with combat and cannot think about anything else. Any NPC is beaten down, searched from head to toe, and his wallet stolen. With the money and ID, the character goes to a bank, steals the guy's money, goes to the Middle East and buys numerous guns, rocket launchers, etc. With their personal arsenal, the character goes on a killing spree, and kills half of the U.S. before getting gunned down in New York, at which point the player hollers and screams and yells at you about being unfair. If you find yourself doing any of these things, stop immediatly. They are extremely unhealthy.
Granted the examples given in the last paragraph are extreme, but necessary. You must understand what these people are like in their extreme forms, so you can get them off the hook early on, before they get this bad. However, where there is evil there is also good, and this is true with gaming. This doesn't change the fact that there is still evil, however. On the whole, there are three major groups of gamers. There are RP Nazis, metagamers, and munchkins. RP Nazis insist on roleplaying everything they can, with no respect for either combat or puzzle solving. Everyone has a little of each. there is a test
here
that you can take if you wish, to find out how much of each you are.