You can start by reading the Swing tutorials, but skip the NetBeans IDE lesson. It's best to begin coding GUIs by hand so you really understand what happens before you let tools take over for you.
Then create a hobby project that you want to make a GUI for. Try to apply the things you've learned in the Swing lessons, and submit your code on our boards for peer review. There's no better way to learn than to listen to constructive criticism.
You need a general grounding in object‑oriented programming and basic understanding of the syntax of Java®. Then find a basic Swing book or the Java™ Tutorials. And make sure to discuss your learning with somebody helpful; you can find such people here