I would definitely say commit your timeline to paper/tracking software etc. I do this to myself all the time where I come up with a timeline and don't stick to it. I found that by having it front of my face (like I usually do at work) it helps me stick to it and let my mind wander less.

Also, I have found that at work it is ok if you get sidetracked sometimes because your guilt or sense of responsibility to your coworkers and company will refocus you soon enough. If you are like me, then you have no such thing when working on your own thing. For that reason, I highly recommend keeping regular, well defined work hours. I would even consider one of those apps that won't let you connect to FB, etc. while you work.

As far as technology goes, you can build a solid prototype website in anything these days. Pick the language you know best and also pick a solid foundation. By that I mean do as little as necessary to make the prototype work. Don't fall into the trap of using this as an excuse to try something new (if you want to actually get it done that is). Frameworks are great for this for example.

Good luck!