When I wanted to start a d20 Modern game with my D&D crowd, the first game I ran was a sort of rules tutorial for everyone (me included). I had everyone make two 10th-level characters, told them to pick one, and tossed them all into an arena with cover, ladders, bridges, and everything else I could think of. Then we all tried to kill each other. I made characters as well, and we dealt with rules questions as a team.
At the end of it, we all knew how firearms worked, how Defense worked (at least, how it was different from normal AC), how the Massive Damage saves worked, and so on.
If you've got people who've played d20 before, a Diplomacy check isn't likely to be too confusing, but doing a new type of combat (moving from D&D to d20 Modern, or Mutants & Masterminds, or BESM) could really throw people off. I strongly recommend having the first few sessions be throwaway combat romps with throwaway characters. This gives everyone the chance to make rules mistakes, character-generation mistakes, and embarrassing goofs in practice sessions instead of messing up their "real" character because of a rules misunderstanding.
Good luck! Have fun!
At the end of it, we all knew how firearms worked, how Defense worked (at least, how it was different from normal AC), how the Massive Damage saves worked, and so on.
If you've got people who've played d20 before, a Diplomacy check isn't likely to be too confusing, but doing a new type of combat (moving from D&D to d20 Modern, or Mutants & Masterminds, or BESM) could really throw people off. I strongly recommend having the first few sessions be throwaway combat romps with throwaway characters. This gives everyone the chance to make rules mistakes, character-generation mistakes, and embarrassing goofs in practice sessions instead of messing up their "real" character because of a rules misunderstanding.
Good luck! Have fun!
