A tap is a device to cut screw threads in a hole. You need to start out with the proper sized hole, or the tap will bind up (too small) or there will be no bite for the screw (too big). You need to know what size hole you now have. Then, by checking what size hole you need for a particular tap, you can figure out what size drill you need for the particular tap and the eventual screw you are going to put in. Taps and screws are designated by their diameter and the number of threads per inch (at least in the USA). Most of the rest of the world uses diameter in metric sizes.
Once you have the proper sized hole, you put some light oil in the hole to lubricate the tap, then, carefully getting the tap straight (note, get a tapered tap - they are much easier to get started straight) turn it about 1/2 turn clockwise, then back it off a little. Continue doing this until you get it deep enough . Backing it off a little with each downward excursion breaks the metal chips off from the cutting edge and helps prevent it from binding up. Note also, that if your hole does not go all the way through, or bottoms out, you may also need a second tap called a bottoming tap. This has threads the full diameter on the bottom edge rather than being tapered (to help start the process) of the tapered tap. You can only use a bottoming tap successfully to start the process if you have something like a drill press or lathe to keep it in alignment; that's why you need the tapered tap to start out the hole. If the hole goes all the way through, and you have enough room to run the tap down far enough to reach the full sized thread cutting surface, you will not need the bottoming tap. It's easier to do than to talk about! By the way, the corresponding part to make a screw is done by the tool called a die. the parts you need to do this are pretty cheap in the small sizes (a few bucks, usually). Just make sure the tap and the screw you buy are the same size! You'll probably want a stainless steel screw - definately something thatwill not corrode.