There is also another can located behind the Johnson house. If you run out a can of spray paint or are busted/killed a new can always be found in the Johnson house save point in the bedroom upstairs. Spraying all the tags will also contribute to getting 100% completion on San Andreas.
It will show you some various examples of where tags may be and what they look like. You will begin tagging Los Santos during the mission “Tagging Up Turf”.