pace yourself, best to start in an open field till you get use to the power of the bike.
do not ride without boots and a helmet. you will want glove and you can just wear jeans instead of riding pants.
and luni gave great advise, get an older like late 90s 250. that way you dont spend a lot and you aint afraid to lay it down. my brother bought a rm250 just for a spare bike to beat the sh*t out of. and that is what we did, we beat it. i know the day he got it, he laid it down on our driveway, he had a nice rash on his ass but the bike was ok. i have laid it down too.