It's a heck lot hard in the beginning.*EnchantingRainbow and ~xSwimmiex have some decent sai tutorials on youtube: www.youtube.com/user/AABloomAA/videos (mine) and www.youtube.com/user/MutantXangel/videos
I actually draw a line with the pencil tool. Then I do a right-mouseclick and choose for 'Stroke Path'. After that, I don't delete the line, but make a new layer underneath the lineart and do a right-mouseclick again and choose for 'Fill Path'.