Make sure you have java installed on your computer. Java is required to run huge games like RuneScape. The easiest way to find out if you have java is to locate C:\Program Files\Java. If it exists, then you do have a version of java and are all set. If not, download the latest version here.
Downloading The Client Edit
3rdScape, and the 3rdScape client are still being worked on right now. We'll have the first public beta done soon, we promise! In the mean time, feel free to offer us suggestions, and ask as many questions as you like.