First, see if your computer is compatible > http://support.apple.com/kb/HT4949
If you haven't purchased OS X Lion yet, you have to call Apple to get OS X Lion > http://support.apple.com/kb/HE57 They will give you a code that you have to insert in the App Store to download Lion.
After downloading Lion, open Finder, select Applications in the sidebar and copy the Lion installer to an external disk, so you won't have to download OS X Lion each time you have to reinstall OS X.
Check that your applications are compatible > http://www.roaringapps.com
Lion will not appear in the App Store. You have to Contact Apple and purchase a download code which they will email to you in a secure PDF. The password to unlock it will be sent in a separate email.
Once you receive the password, open the PDF, copy the download code, and subsequently "redeem" it in the App Store to start your Lion download.