The agent listed on the sign works for the home owner. They have signed an agreement with them to get the best deal they can. (Best can be measured by money, speed, hassle).
If you use the same agent, they probably will have to give you a dual agent form, which advises you of that fact, and you recognize they work for two parties with different goals.
Therefore get your own agent. They will show you houses that you are interested in and can afford. They will keep your interests in mind, they can help you determine the amount your should offer, and the contingencies you should put on the deal. Remember the seller wants the most money and no contingencies.
If there is one agent, they get the full commission (lets assume 6%). If each party has their own agent, they will each get 3%. The commission is paid for from the sellers funds. (Yes I know the commission is rolled into the price/mortgage). Having your own agent will not increase the buyers costs.
There are times when a sellers agent tries to keep the listing for themselves, so they get the full commission. They are hurting the seller when they do this. They are limiting the possible buyers, they are telling anybody with an agent to stay away.