If you highlight your hard drive icon on your desk top and press 'apple' and 'I' it will give you a breakdown of how much of your hard drive is being used.
Like wise if you highlight any application or folder and press 'apple' + 'I' it will tell you how much memory it is using.
Delete the memory hungry applications or folders that you never use.
Also empty your trash - trash still uses up memory until its emptied.
Do you use i-photo? Saved pictures use up loads of memory. Either invest in a external hard drive (i bought one for about 70 quid) or burn your photos to a disk and delete them.
I would say the safest and easiset way is to have a good clear out of your internal hard drive and think about investing in an external hard drive.
This is the one i use:
http://store.apple.com/Apple/WebObjects/ukstor e.woa/6464042/wo/8C5h6ofBsrne2qdtuiJ18I1FSxP/7 .PSLID?mco=16A083B2&nplm=T4674