The guide MustISO linked is only good for 10.5 and earlier.
For 10.6 (10.7 is via the recovery partition), by default, normal installations in 10.6 will only do Archive and Install, preserving all of your current data. You need to manually remove your old installation. 
When you get to the installation, before you start selecting a drive, look at the menu bar. It should say "Utilities" on it. In utilities, there's Disk Utility. Open Disk Utility, select the drive, and in the main part of the window, it should say "Erase" in a tab. Select it, and you can erase. The options available are zero-passes (1-10 I think), and what FS the newly formatted partition will be.
Please note this WILL ERASE ALL OF YOUR DATA so back up accordingly.
Then, quit Disk Utility and continue your installation as you have, and it will be 100% fresh.