The pubococcygeus muscles (PC muscles) are involved in a variety of important functions related to the pelvic floor and tied to sexual response and pleasure. Many people begin doing Kegel exercises, which are exercises designed to strengthen the PC muscles, but are actually squeezing the wrong muscle. It's important to make sure you are exercising the correct muscle as doing exercises incorrectly can be a waste of time and they could actually worsen a situation.
The best way to find your PC muscle is to insert one finger into the vagina and try to squeeze down around your finger. When you feel your muscles tightening around your finger you're squeezing your PC muscle. You may also feel a kind of lifting in that area as you squeeze and then a dropping down as you release. If you don't have a vagina you can also try putting the tip of your finger at your anus. Imaging squeezing the muscle you use to stop the flow of urine when you are peeing. You should be able to feel both the tightening and relaxation at the anal opening with your finger. Once you can feel what it's like to tighten and release you know how to control the PC muscle.
Because the PC muscles are used to stop the flow of urine, a common suggestion to find the right muscle is to stop the flow of urine while you're peeing and take note of what that feels like and where you feel it. Doing this often or for prolonged periods of time isn't a good idea though. If using your finger works that's better. If it doesn't, just use the stop-start pee technique a few times, but don't continue it as part of your regular exercises.
If you are looking for your PC muscle because of a medical concern or condition it is highly recommended that you consult with a health care practitioner, preferably one who has expertise in the pelvic floor. Instructions like these online can help in general, but every body is different and if one-on-one consultation is possible, it is usually preferable.