Assuming you want to clean install (follow these steps; I just did this a few hours ago).
1. BACK UP ANYTHING IMPORTANT!!!! I forgot to backup my music (luckily I found a free program to copy all my music back from my iPod or else I was screwed!) BACK IT UP NOW! If you don't want to do a Time Machine file, then just copy your files that you want (E.G. iTunes Folder, important documents).
2. Restart/boot-up your computer. Press and hold Command+R until a menu comes up asking you what language you want. Choose what Language you want and hit continue.
3. Choose "Disk Ultility". Choose your Mac OS X partition, choose the "Erase" tab, then choose "Erase"
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4. Exit out of the Disk Utility and Choose "Reinstall Lion".
5. Follow the instructions given.
Hope I helped!