The following is a list of how to get a 100% completion:

Successfully complete all 90 story and procedural missions to get 68% complete.
Successfully complete all 30 vehicle thefts to get an additional 2%.
Successfully complete all 50 stunt jumps to get an additional 2.5%.
Collect all 200 pigeons (flying rats) to get an additional 2.5%.
Find all random characters and complete their missions to get an additional 5%.
Successfully complete all 20 vigilante crimes to get an additional 2.5%.
Kill all 30 people on the list to get an additional 2.5%.
Beat friends or computers at every activity to get an additional 5%.
Get all your friend's special abilities and do all their activities to get an additional 10%. Note: This does not include Dwayne or Roman.