Sneak up behind him and yell, Boooooooooo!....

Just kidding, In your script you need to assign code and use ent_animate.

eg:
if (key_space)
{
c_move(my,vector(0,0,10),nullvector,IGNORE_PASSABLE);
ent_animate(my,"jump",5,null);
}

Don't use the above code as it hasn't been tested..

Refer to manual for explanations, and do a search in the forums for player movement or look on the wiki site...

Good luck..