If your air conditioner will turn on, and your thermostat is set properly, but your system is not cooling you may have a dirty or blocked air condenser. Your first step is to check the outside unit and clean around and remove any debris or weeds that are blocking air circulation.
Next, check your filter and make sure it is clean. A lot of times, we find the filter is dirty, and when it restricts the air to the evaporator coil, the coil can freeze. If you have pets or run your a/c often, you may need to change your filter more frequently. A good test is to pull your filter out and check if you can see through it.
If your air conditioner is still not cooling air, you may have a problem with your compressor or refrigerant and will need to contact a professional.