Many places will test it for free and suggest some chemicals. The first thing you will need is chlorine. Chlorine kills any bacteria that may be in your pool water and that is why it is so important. Shock your pool once a week in the summer. This basically means, give your pool a chlorine overdose to kill anything that might be in there.
Shock usually comes in two varieties, liquid and powder.
Both will do the job, however powder shock can sometimes cloud up some pools. Make sure you are using the right kind for your type of pool. Read the directions and when in doubt, ask the pool shop. It is also a good idea to shock the pool in the evening. Direct sunlight can strip your pool of chlorine before it has really had a chance to work.