Here's a list of things to do, and I recommend you do them all, in order.
1) Download
MacScan and have it scan your Mac, just in case you have some sort of trojan or spyware. If you do, have MacScan rid your system of the infected files.
2) Download
OnyX and allow it to check the S.M.A.R.T status and verify the startup volume when you open the app. (It will ask to do these things automatically.) Run everything in the Maintenance and Cleaning sections of the application.
3) Delete applications you do not need. Unfortunately, you do not entirely remove apps when you drag them to the Trash - they leave other files behind, and depending on what app we're talking about, those files can take up a rather significant amount of space and memory. Download
CleanApp to remove all of these extra files, as well as other large files on your disk that haven't been used in a long time.
4) Reinstall Mac OS X. I know it's not fun to do, but it's really something that should be done every now and then to ensure your system keeps up to par. I reinstall mine every few months, and I never notice and lag or sluggishness on any of my Macs. Just use the OS X Install Disks that came with your Mac. You can find instructions for reinstalling the OS, depending on how you want to do it,
here.