When you go to your first class, be prepared to be totally lost. Even a 'beginning' class is designed for continuining students who already know the basic terminology. However, after a few classes, you'll start picking it up and seeing why so many of us enjoy ballet even if we'll never be Baryshnikov.
Adult drop-in classes tend to be more free-form with their dress code than pre-professional academies, but you can't go wrong showing up in the classic male uniform: black tights/white t-shirt/dance belt/slippers. Your slippers can be either white or black, but white ones with thin white socks give the instructor a better view of your feet to tell what you are doing wrong. That's also one of the reasons for tights - so the teacher can see accurately where your "line" is and give appropriate corrections. So even if you can get away with wearing sweat pants to class, wearing tights will help you improve faster. Dance belts are pretty strange feeling at first, but necessary, so get used to it.
There are several online web stores that sell male dancewear, but if you have access to a big-city storefront, you'll be able to try different sizes. This is important because dancewear sizes don't seem to have any correlation to normal clothing sizes. I've got an 8.5D normal shoe size, which somehow translates into a 7B ballet shoe. Go figure. If you can't find the stuff you need locally, at least order from a mail order place where you can discuss sizing with a real human being.
Good luck and feel free to write if you have other questions.
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