want to build a 2 channel system where do i start

Go out and listen! There is all sorts of theory, favorites, opinions but only one set of your ears, With that sort of budget you can surely attempt an in home demo and I would atleast look into that, I can say you may be steered in better direction by listing room size, size of speaker preference, your location so clsoe dealers can be factored in for the lines they carry among other ideas.

I agree with Chadliz, but you must also decide what you want from a system. Do you want a turntable, do you mind lots of components or do you prefer to keep it simple. if it was up to me, I would devide my money like this more or less.
Speakers $8k to $10k
amp $5k to $7k
CD $1k to $3k
cabels $0 to $2k

It get more complicated if you want a turntable as well. But just buying components won't make up for a good system. I would start with speakers. Listen to cheap and expenive speakers, from bookshelf to floorstanding. Figure out what kind of sound you like. Then start to listen to speakers that have that sound within you price range. Next find an amp that can drive those speakers without making them sound bad. Next buy a CD player. I think the CD player is the least impotand step. Sure there are differances between playes but not so much as between speakers.
I own a CEC TL51x with Wadia 12 DAC. I think this combo would no sound bad in a $20k system.

With the amount of money you want to spend ($20,000) you should be able to built quite a nice system. If it was me with that much money and wanting to know where to start, I find a good, well-established, hi-end dealer with a good reputation for sales and service, and between the two of you build a system that meets you listening wants and needs. Granted it will cost more to go this route, than buying used, but then again you're paying for the dealer's experience and expertise, and a good hi-end dealer can help you avoid many expensive mistakes in putting together a good system. Of course, this advice is considered somewhat "heresy" over here on Audiogon, as many of the people here are seriously into the "buy/sell" game, and would like nothing better than getting another "player" into the game. One more piece of advice, remember in the final analysis, it's about the music, and not so much the gear. The gear itself is just a tool to get you to the music. Unfortunately, there seems to be many that have forgotten that fact.