Look around you. You may have a common name like John, Alice , Catherine. There are names that are very popular and so are their nicks. If you want to be on the safe side, adopt one of them.
Names in your name:
Write your name down on a piece of paper. Try to make names out of it. Cut it short like cutting William to Willi Alexander for example could be shortened to Alex, a very decent nick.
When you select a nick, speak it out aloud and judge how it sounds. Do some practice with pronouncing it as any friend of yours will do. If it sounds too awkward or hard to pronounce, do not go for it. Ask a family member and close friend about their opinion of the name and ask them to call you a few times by using it. If you like how it sounds, go for it.
Online Nick generators:
There are online websites that offer to choose a nick for you based on your name. They have a large database of nicks used around the world and give you several options to choose from. Some software are so advanced that they decide your nick based on not only your name but also the personality description that you give them.