Need help with Horseback Riding?

Ok, so I'm totally freaking out because my friend's birthday party is tomorrow and I have never ridden a horse. Also, my Dad has to sign a wager or something so if I get hurt, we don't sue or something. And I am the biggest klutz alive! Also, what do you wear to go horse back riding? Are jeans ok? And converse? Can I not wear loose clothing? Just please give me all the basics to horseback riding pleas.:) Thank you! Fritos!