dont look now Asked: My mother is "making" me go to a horse riding lesson, need a few tips, help?

I'm not that into horses, I'm more of a dog-person, not a horse-person, but my Mum is absolutely horse crazy! She loves horses, too bad she doesn't have enough money to own one, but she does volunteer at a local riding stable that's only a few minutes drive from our house.

She signed me up for a lesson without telling me, now I'm super nervous! I've never even touched a horse before! And I don't know ANYTHING about horses. I've never read about them, I've never watched The Saddle Club or any other horsey TV shows, so I hardly even know HOW to ride a horse!

I'm not really certain of what to wear, the lesson is on Sunday, this week and I'm REALLY stressed out! Is it okay if I wear jeans? Do I need to buy a riding hat or helmet or whatever they are called on e-bay or something?! Do I need to buy the saddle for the horse? What if I fall off the horse and get myself hurt?
And also, do they let you chose your own horse, or they chose the horse for you? :$
Any advice is appreciated, I'm sure you horsey people in the Horse Section could make a real help to me! Thanks!