how to get an autograph from active sim

Just go to household management, move in a random sim and have your celebrities write autographs for that sim. You can then move the autograph to your sim's inventory from theirs. Finally move the random sim back into the world (and give them back their money).

Or use the cheat menu, there should be "generate autograph" somewhere nested in the reputation menu and it also lets you choose what kind of autograph you want (framed, paper).