Buy a refrigerant recharge kit(bottle&gauge/hose)...sold at auto parts stores, also sold at walmart, priced from & $20-$30 a kit. It comes with instructions,which easy to understand.
Yes you will need the compressor and fan running while you add refrigerant(freon is not sold anymore).
Its simple when youre reading the instructions. I've done in 2 vehicles in which one ended up in the a/c shop because its improperly recharged. So I would discourage first timer to service a/c...you might end up spending more.