[RogueCoder]

I suggest you start looking into sql injection and cross-site scripting. These are the most common vulnerabilities even though sql injections are getting less common these days, there's still a lot of vulnerable sites out there

You should read these tutorials
My SQL injection complete tutorial by @V1P3R
Basic xss tutorial by @Anima Templi

To get some real hands on practice you should check out the "Targets for training" and "Self-Hosted Vulnerable Web Applications" sections here

You should also learn; HTML and JavaScript. This will give you a basic foundation to continue to build on