My biggest recommendation to you is wear what you'll be comfortable in. If you're not comfortable in a tank top feel free to wear a loose tshirt. Especially when you're just getting used to working out in a gym setting it's important for you to feel at ease. For shoes you want something comfortable for standing in with good traction. I think gyms may prefer non dark soles which can scuff the floor but feel free to ask.
As for food it really depends on what time of day you're working out at, what kind of activity you'll be doing and things like that. I would think at entry level how ever you normally eat would be fine just don't go to the class with a really full belly and don't be suprised if you're extra hungry afterwards. Just try to remember to keep it healthy for the rest of the day.
Good luck!!