A word of caution, many times, projects that start like that never get finished. I have seen it time after time, and I have made the same mistake. A lot of the time, when you "learn as you go" you just get frustrated and give up.

I strongly advise you to start small, and work from there, as Ulillillia suggested as well. You do not have to follow this advice, but I am simply presenting you with my knowledge of the subject as seen in this forum.

Scripting is pretty much the hardest part of game design, for me. Second comes texture creation, then modeling, then general level design. I suggest you figure out what the area of most difficulty is for you in game design, and start learning about it. Then, after you have a good grasp, go out and start building tests, demos, whatever you need to help you learn.

Again, you could very well start your doom project now, but I fear that it will not get finished.