By biggest advice to you is to do yoga and pilates every day. If you can't afford actual classes, buy an at home DVD and do the strengthening, stretching, and core strength exercises. Ballet dancers don't get their powerful bodies from ballet - they get it from yoga, Pilates, and various other cross training methods like swimming.
If you're only taking one class a week, you shouldn't be en pointe for at least 3 years. You need a strong fundamental basis before you can advance to that level. Any sooner would be incredibly dangerous to you.
First class will probably be a series of 'easy' exercises so the teacher can assess everybody's skill level.
Bring (well, wear) a black leotard with ballet pink tights unless your studio specifies other colors. Full soled leather ballet slippers are my personal preference for beginner level students as they help strengthen the feet. You may bring short shorts or a wrap skirt if your studio allows. Also bring a water bottle, granola bar, and notebook to write down new vocabulary words (ballet is mostly in French so it's good to know the literal translation of things). Invest in a small bag or backpack to keep your dance things in.