Confused about SATs! need help!?

ok well im only a freshman but i have older cousins and friends that have been taking the SATs and i was just wondering how this works! So do u take the PSAT in 10th grade? and i know that they r practice ones! So then in 11th or 12 th grade u can take SATs? right? and lets say that you didnt do good in the 1st try so can u take it again? how many times can u take it again? and what score do they count? the 1st time you took it or the second time? can u please tell me the highest score you can get on each of the subjects? and what subjects are the SATs on? thanks!