depending on when you got it, it probobly only has 512 megs of ram. that would be a great place to start.....buy yourself like 2 gigs of memory. you can get 2 gig sticks for like $50 and they're SUPER easy to put in. 2 gigs is the max for macbooks, so don't go buy like 2 2 gig sticks and think that you can run both.
also, i don't know if you've been a mac user for a long time, but i've seen people do this a lot....make sure you are exiting your programs and not just closing the window. actually go to the drop down at the top of the screen and click "exit", don't just click the little red x. seems retarded, but i've seen soooo many people do this. you don't want multiple applications running at once. you could also just use apple + q to quit programs faster.
get random shit off of your desktop. you shouldn't have any more than 10 icons. and keep your dock small, with as few icons as possible. just get rid of icons and use spotlight all the time.
go through your applications folder and delete random shit you don't use.