Gretchen, general first aid guidelines for snake bites advise having the person rest in a position where the bitten area is level with or lower than the heart. If possible, the area should be immobilized to prevent movement. This slows the spread of the poison through the body.
The specific first aid measures depend on the type of snake bite. With snakes that release venom that destroys tissue at the bite site, it would be best for the person to be in a neutral position with the bitten area at the same level as the heart.
Antivenom is the definitive treatment for venomous snakebites and should be administered as soon as possible after a bite. As a first aider, the best thing you can do is call EMS immediately, have the person rest comfortably, and provide reassurance.