Use comfortable clothes that make you free to move. Being comfortable is far more important than looking good! At most places can you borrow or hire a mat if you don’t have your own. Avoid to eat a big meal just before class, a snack or some tea is okay, but you don’t want to be to filled up when doing poses like twists and inversions.
It is totally fine to feel a bit lost and confused at your first class. Try not to judge and compare yourself to other students. There will always be people in the class who seem to know what they are doing – but never forget, they were once you!
Be open-minded. Always keep in mind, there are always more styles and teachers to try if the first try wasn’t a very good match. Save questions to after class. Always respect the teacher and the other students in the room.