I would like to buy a house. I drove by the house and saw an agents name on the sign. Do I call them and start negotiating?

If you don't have your own agent, the seller's agent will often represent both you and the seller as a "dual agent" or just represent the seller. This means the agent either has divided loyalties or is working for the seller, not you. I would strongly suggest using the services of a real estate agent to represent you. And remember, the seller pays the real estate commission, not the buyer.