Summarize this article:

There are two types of cat harness: the Figure Eight and the H-harness. The main difference between the two is that the Figure Eight features just two loops that meet on top of the cat's shoulders, while the H-harness has a short strap that lies between the shoulder blades with the loops sewn onto either end. There is no right answer to which type of harness is best. Some owners report the Figure Eight harness is harder to wriggle out of. Indeed, the fit of the Figure Eight harness is much more snug, which could account for why it seems more secure on some cats. Harnesses usually come in different sizes, such as small, medium, or large. These sizes are typically based on the measurement around the chest, usually 12, 14, 16, or 18 inches.  Most harnesses have adjustable loops but they are not designed so that a small harness can fit a big cat. Rather, these are designed for ease of comfort and to get the harness to fit snugly. To find your cat's chest size use a tape measure and place it immediately behind the cat's front legs. Measure around their chest in a full circle, taking care that the tape is not twisted. Apply gentle pressure so the tape fits snugly but is not digging into the cat. Take this measurement and add 2 or 3 inches - this figure now gives you the size of harness you should buy. Cat harnesses are available at most pet stores and they come in a variety of colors and materials. If you have a specific model in mind, however, it may be easiest to buy the harness online. Remember that sizes may vary between brands, so a Large in one brand of harness may not fit the same as a Large in another brand.

Summary:
Pick a type of cat harness. Figure out what size harness your cat will need. Buy a harness for your cat.