Dont buy anything on your first trip, dont even bring spending money (except for admission). Go to the market, go thru the bins and binders and cabinets. See what you like. Ask lots of questions. Write down sample prices. Some dealera specialize in different countries / periods / metals etc. Spend a few hours there. It's free education.
Then go home and compare what you saw with prices on the Internet (ebay, NGC, PCGS, etc).
In all cases beware of fakes / counterfeits / copies. If you're new you wont know whats real, even a pro gets fooled once in awhile. If you have the resources you can bring a pocket scale and a loupe to compare the coin with on-line photos and dimensions. If nothing else, bring a magnet! Silver is not magnetic.
Finally, if the deal looks too good to be true then it's not true! You wont find a proof Morgan dollar or gold doubloon for 5 bucks. Those are made in China. But the counterfeiters are smart so they know how to “age” coins and use tin etc to make coins look real. If you're not 100% sure, walk away! Always always be ready to walk away.