Okay, if you are looking for a game engine (map editor and run-time core) I donât knwo what you can find, go search Sourceforge at a guess.
If you want to actually write your own game code, you need at least the Java2 SDK. You can download that at java.sun.com. That plus any text editor will let you write and compile Java code including Applets.
Its easier to write in an IDE and there are many available, some of the most popular free ones are Sunâs Forte (or Sun One Studio for Java, or whatever weâre calling it now), NetBeans (which is the generic OpenSource version of the Sun IDE), Eclipse (which IBM pushes) and JBuilder Personal (which you cna download from the Borland site.)
If you want to run your Applet on MSVM you are going to have to code to the 1.1.8 standard NOT the current 1.4 standard. This means being careful about what classes and methods you use and compiling with the proper flags to generate 1.1.8 compatible class files. Once you get into that folks here can help point you at instructions on how to do that.
Keep in mind that Java is a real general purpose programming language, as opposed to Flash/Lingo which is really a hyper-evolved multi-media scripting language. This means you can do a lot more in java but the learning curve is going to be greater and doing simple multi-media type things are going to take a little more work. What you gain for that is the ability to do complex things not possible in Flash.
If you have little to no general purpose programming experience I would start with a MUCH less ambitious project. A fairly single player game is a good starting point to learn to code Applet games. (eg pac man, Defender, that sort of thing.)