Well there's a second on ELO in the rules that explains this.
But the lowdown is:
The total stars (earned not on the card itself) must be less than or equal to 25 stars across your entire team.
You cannot have duplicate characters (ie you can't run 2 Jackies)
You cannot have DjKorr, Kiki, or General on your team. (I'm certain of the first 2, I think General is the 3rd. Again check the rules).
You must be level 20 or higher.