Start by scooping out as much debris as you can. Hook up the pump and filter and start them up. Add about 2 gallons of plain, unscented 6% bleach slowly to the skimmer while the pump is running. Run the pump 24/7 and backwash it when the pressure rises 5-8psi over your clean filter pressure. Add more bleach each evening.

Can you go to a pool store and have them test your water? Just don't let them talk you into buying anything til you post the results here on the forum. After we look at your numbers, we give you better advice. (Get yourself a good test kit if you can. We recommend the Taylor K-2006 or 2006C - see Amazon link below).