Try a variety. Some brewpubs will offer a "beer flight" or other deal where you get, for one price, 6 oz glasses of a variety of different beers. It's a good way to sample lots of different styles and see what you dig.
That said, anything you don't like, be sure to revisit later. My first Guinness I loathed. After a few years my tastes changed and now I'm really into rich, dark, malty beers like that. There are good American beers, but you'll have to dodge the megabreweries.
I don't know your region, but I'm in the South. Abita, from Abita Springs, Louisiana, makes some really good product -- Purple Haze is a berry-tasting light wheat beer that is well received, while Turbodog is a hoppy dark ale with a goodly amount of bitter to it, and their Restoration Ale gives you a charitable rationalization for drinking beer (proceeds go towards Katrina rebuilding). Dixie's Crimson Voodoo and Blackened Voodoo are both, to me, delightful if a bit hard to find. Olde Towne Brewery in Huntsville, AL makes a good Bock (that sometimes has an extra kick) and a really interesting Hefeweizen. Sweet Water in Atlanta has a really neato blueberry ale, and their other offerings have generally been nice. If you can get local product, then do so -- it's interesting to taste what the locals are brewing up.
You'd probably do well to have a friend along who knows the various types of beers -- for instance, all Belgian whita ales (example - Blue Moon) will be somewhat similar. All Bocks will be somewhat similar, et c -- these are classes of beer. You might enjoy a trip to a high-end beer store that sells beers a la carte -- World Market, in a pinch, or a beer superstore, will often let you "build your own six-pack". It costs a little more but you get to sample lots of things.
Just please, please please, don't drink the beers America's infamous for. This country CAN make good beer, we just seem to prefer CHEAP beer.
