An applet is a miniature program written in the JAVA language, which is displayed within a browser.
Use the Applets Tutorial after you have learned the basic syntax of JAVA. (Look under Tutorials / JAVA for the tutorial.) There's no shortcuts to learning, but there are some types of code that you don't have to learn to write simple applets. Just make sure you know the basics. Importing classes, declaring variables, passing parameters to methods, variable scope, etc. If you don't understand something right off hand, don't worry. My experience has been that a light will come on later if you set the problem aside, and continue on using the examples given.
If you have a question you have two choices.