Welcome to HTS, I'd advice you that while doing the missions you should pick a programming language and learn it.
You'll already have figuered out that basic HTML was a requirement for the basic missions. The realistic ones rely more on php with SQL so those are good things to start with. (note: noone will ever agree on what language is the best to start with, so don't let this post or the following limit you, learn whatever you want!)
Above all, have fun. The missions are there to guide you to learn new things, but theres no real limit on what you can start off with. Pick a topic, read learn and try it out. (And as pointed out by our mod, browse the forum, theres a lot of threads on what to start with all with totally different answers
Happy hacking ^^