Go to the apple menu and take Software Update. (This may take some time depending on your internet connection). Your Mac will automatically download all the latest Apple software (OS, iLife, iWork, QuickTime, etc). It may ask you a couple of questions about installing itself. Then repeat this process once more as some fixes require secondary fixes (10.5.2 & Graphics system update for instance) and then you'll be good to go.
24" iMac, 2.8 GHz, 4 GB RAM, 1 TB HD
eMac, 700Mz, 640 MB RAM, 40 GB HD