Garter snakes are harmless, non-venomous snakes. They can be very helpful in controlling mouse or insect problems. But, if you really want to get rid of them, first minimize their habitat by planting bushes away from the house and keeping them trimmed back, and keeping your lawn mowed short. Also, remove their food source by treating your home and yard for rodents and bugs. Snake repellents can also be helpful, but are most successful when used in specific small areas. If all else fails, contact an exterminator and let him deal with it.