You can purchase, download, and install v10.7 Lion from the App Store.
You can launch the App Store from your Apple menu, your Dock, or your Applications folder.
Make sure your Mac meets Lion's requirements ..
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OSX v10.6.6 or later (V10.6.8 recommended)
- 7GB of available spare
From here > Apple - OS X Lion - Technical specifications
Read before upgrading to Lion > Lion upgrade questions: Apple Support Communities