There are numbers of companies to help set up a merchant account for a US citizen - Chase / Paymentech, First Data, CDG Commerce. And then you will need an electronic payment gateway (LinkPoint, Authorizenet.com, Verisign's Payflow). These will communicate to the transaction processor (First Data / Nova, etc). If you are doing it less than $1,000 a month, consider Paypal or 2CO. Usually it is cheaper - especially on your time. (most people never consider their time as money.) If you need a seasonal merchant account, First Data offers these. If you only need it for a few months, talk to Adam over at CDGCommerce.
Being in the US you have a lot of options available to you. The only real issue you will have is because you will have so many options you might find it hard to compare all of them. Especially if you don't know what to to look for. Use this merchant account provider comparison checklist to help you organize your information and make a better comparison. If I may ask, What kind of products do you intend to sell? How much do you expect to make per month?
Paypal has its place but it is not the best for all merchants. In fact, it typically is best for smaller merchants as larger merchants and merchants serious about their customers' perception of their business will always choose a true merchant account.