Affiliate Marketing Secrets

I’m interested in learning some web design, but I’m not sure where I should focus my efforts. I’m interested in learning how to develop web applications – the kind of thing that would allow me to develop interactive applications, interact with databases, etc.
I’ve got a bit of background in programming, but it’s dated. I also have a stats/econ background, and I’m hoping to pick up enough web skills to eventually develop some niche online tools relating to statistical analysis and economics (and hopefully generate some side income from these tools).
What kind of things would I need to learn to do this (software/languages, courses, etc.) ? How much work is it to get a basic competency in these areas?