Prototyping fast, then start the second edition. Don't wait until you are confident enough to start. I find that whatever I do whatever language I use, I always do the best in the second version.
Next semester, I'm in an advanced PHP course - enough to really firm up the things I'm working on. I'll probably have a prototype made winter break, so I'll be able to fix up the beta and stabilize it as part of that class, and have a working model in time to apply for YCombinator this summer.
I'm working with other PHP projects so I can get confident enough in PHP to make my site. The code for it is really simple: one of the things that makes me certain about my idea is that it's an incredibly basic one that hasn't been used in the way I'll be using it. I'm not too worried about it getting complex, so I'll know exactly when I'm ready to dash it out. But HTML first.
YMMV. It helps if you use a framework or something else that can get you started. Just remember that customizing a framework isn't your core competency.
The UI elements are pretty much set. This is an idea that's been a long time coming, and we've got ourselves almost entirely planned out.