I found that before I started joining classes it was really beneficial to try some home yoga sequences just so that I didn't feel shy or overwhelmed when I was surrounded by people.
You really want to get yourself a nice, comfortable pair of yoga pants (best investment I made), what you wear on top doesn't really matter too much as long as it doesn't flop over your head when you do poses like downward facing dog.
And yes definitely drink a bit of water (not enough to get a stitch) if you need it.
Also if you don't enjoy classes it's not the end of the world, don't give up on yoga all together. I didn't enjoy like the group yoga vibe so I just started doing it every day on my own and my practice has grown amazingly since then.