You need to check to see if your machine can handle Lion, it needs 2GB (4GB preferred) and a Intel Core 2 Duo or better.
Download the free MacTracker and see more specifics of everything.
http://mactracker.ca/
Next, you must realize that your Rosetta PPC based programs will not operate under Lion, likely you will have to buy all new programs.
If you machine is dated, a lot of the trackpad gesture features you won't have.
Check the source of all your third party sofftware and hardware drivers for scanners, all-in-one's and the like to make sure they work or there are updates, or upgrades available and how much.
http://roaringapps.com/apps:table
IDEALLY, it's best to backup files and then "fresh install" Snow Leopard clean, then upgrade to Lion, then install programs and finally files from backup storage drive (not TimeMachine!)
Also you should make a option bootable clone of Snow Leopard on a external drive, this will allow you to return if something doesn't work.
You can learn all of this and more at my exhaustive post here, enjoy.
https://discussions.apple.com/thread/3358920