This all varies greatly with exactly what you are trying to do. Payment gateways are very security sensitive because you are dealing with money. There are usually pre-made scripts that can do most of the work for you but some knowledge is still required just to set it up. Google is your friend here.

Your fastest option may be to hire someone, you can explain your needs to them and they will either build you something or use pre-made scripts and customize them as necessary. With this you get what you pay for, the cheaper the worker the lower quality you'll get. You'll see a lot of offshore consulting companies from developing nations like India and Pakistan who charge a fraction of what consultants from developed countries cost, but the work you get will be on par with the pay.