Spamassassian Question....

Hi All,

I'm new to Spamassassian. I just activated it on my server. I am
receiving a lot of spam. Lots of these letters are those stupid ones
that have subjects like: "Hi" or "You won a new laptop", etc.......

How can I write a custom ruleset to weed out the letters which I do not
want to receive anymore???

I'm kind of lost......

Thanks for your help.