Distinguishing the genders of unaltered adult cats is rarely difficult, but kittens are a different story. Doing so takes a bit of practice; you might not always be right, even after you've gained some experience. However, having an idea whether a kitten is a boy or girl satisfies curiosity and helps you choose a name. You'll get confirmation when the kitten sees the vet for vaccinations and neutering.
Check the kitten's fur. Tortoiseshell cats (those with a mottled pattern of orange, white and black) are almost always girls; kittens with ginger fur are usually boys. Other colours and patterns could be either.
Wash your hands before handling the kitten, and again afterward.
Place the towel in your lap and pick the kitten up with both hands.
Place the kitten on its front on the towel. Gently lift the tail up.
Look at the two openings. The one nearest the base of the tail is the anus. The other is reproductive in nature. Male kittens have the openings further apart than females; the shape also differs. In boys the reproductive opening is circular; in females it is a slit. You won't see typical male genitalia in kittens, but if you feel gently you may be able to detect the testicles in kittens older than two months --- they'll feel like peas under the skin above the reproductive opening. In older male cats they are normally obvious on sight.
Avoid handling newborn kittens. Wait until the kitten is at least three weeks old before trying to sex it. Before this, you will upset, and may damage, the kitten, or even cause the mother to reject it. At this age, you probably won't be able to tell the gender anyway. If the kitten or the mother makes distress calls, put the kitten back. You can sex the kitten a week later or so, when it is more confident.