Knock down any mud tunnels that you see. While this does not get rid of the actual infestation, it does cut off traveling termite pathways for a while.
Call an inspector. Ask for his appraisal of your infestation and for an estimate of how much it would cost to treat the infestation. Do not commit to an inspector in a panic; instead, shop around, and determine who can offer you the best deal. Your house has probably been infested for years and is not going to fall down tomorrow.
Remove any sources of moisture around the home that may be supporting termites. Reroute drains and gutters to direct water away from the home. Make sure mulches do not touch your home.