First remove your files by backing up a copy to a external drive.
Then reboot holding the Command and R keys to enter Lion Recovery Partition, your going to need a Internet connection, Ethernet cable is preferred and easier than Wifi.
If you just need to reinstall OS X Lion you can, your files and most programs should be fine, if you want to wipe the OS X Lion then select Disk Utility and format the OS X partition HFS+ Journaled and click erase, (with security option zero, 7 pass etc for completely overwriting all data)
Once that's completed quit Disk Utility and reinstall OS X Lion, it will have to be downloaded from the Internet so a good strong fast connection and some time to wait is required.
Later on you might want to consider cloning OS X using Carbon Copy Cloner or making Lion boot DVD's, or buying the Apple Lion USB installer for $69 or so.