Clean up mac os x 10.5.8

hello there,

i need a tip or more tips? 😉

my imac seems to freeze all the time especially in the internet...

so my question is:
i remember from school (200 yrs ago ;-)) that there were a few keyboard shortcuts for monthly or weekly so called clean ups or starting up without preferences or similar and i really forgot that.

could anyone like post me a list of things to keep my "baby" maintained and well working?

thanks guys!