You should see your GP.. there's a few creams and things you can try - stuff that you use on your face for pimples works on backs too, but it can be expensive. Cetaphil is awesome for that, you can get it in large tubs...
RE: the pill.. depression is a side effect but it's not especially common. going on the pill helps lots of women with acne (not all though), particularly if its especially hormone related. theres some different kinds of pills you can take like Diane which has more of an anti-testosterone effect.
exfoliation is also good, lots of beauty salons now offer back treatments and things which are like facials. or you can buy some deep cleansers (oil free) and put them on your skin.. make sure you exfoliate regularly.. drink lots of water and eat healthily
theres other pills you can take for severe acne, like roaccutane, but you need to be assessed by a dermatologist first. after you try basic things, ask your gp to refer you to a dermatologist for advice.
hopefully i helped a little
good first port of call is a beauty salon, chemist or a GP.