It depends on how old they are, as tiny children - some people start taking their babies in the water when they are only a year old - will probably have no fear of the water, while if they are older they may be nervous at first.

Assuming your child is over about 3 years, the main thing is to teach him or her not to be afraid. Of course there may be no problem anyway - I was never afraid of the water, were you? But in any case, you should get in the water with your child, let them play within their depth and then start floating, with armband or rubber rings to start with. They should get on their backs as soon as possible, so that they feel comfortable with floating on the water, and then you can show them the first simple arm and leg movements (this can also be done separately, out of the water, before you start practising in the water.)

You could start with this free online course to help you teach your child.