What's wrong with my suspension?

so i have a 96 pontiac firebird, 3.8l. I bought it from someone so i didnt do any of the work done to it. my problem is when i go over bumps or back out of a driveway on a sloop i either scrap or in the case of a bump smash onto the ground, i guess "bottom out". it gets worse the more weight i have in my car. I had me and 3 other average size people in my car and i tried to drive, i was scrapping on the ground the entire time. something has to be wrong with my suspension, either shocks or springs or something.. and even with 3 people in my car, if i accelerate, there is a loud rubbing noise that comes from the back of the car, and you can feel it. what is wrong?