Well, first you'll need to learn Javascript, Flash, etc. Sorry, I can't really help, I'm not good at programing.
Everybody should have Java. I have it on my computer.
But first you need to know Java to make it. currently, I am working my way up to Java. I have used 10 different programming languages (some of them I dont know very well though)
well you dont need it, but it's a good idea, almost all computers have it, it can be web based and it is a good programing language. also runescape was made with java.