First thing
The first thing that I did was to get a parts diagram.
These are available online at Chestnutridge.com or in most of the technical manuals and such.
Then look at the links portion of this forum for suppliers. Also can search online using m14/m1a and lots of links will come up.
Secure a good bolt, op rod, trigger group, and gas cylinder first. Then select your reciever (get a LRB if you can afford it, but fulton, armscorp, and poly are good too. there are others.). Decide on what barrel you want (USGI chrome, standard or med or heavy, commercial standard or NM, etc..). Then worry about the smaller parts. Then select a stock - see FredsM14stocks for a good discussion regarding difference between wood and synthetic. If you get USGI parts, you will have a good start. NM mods will then be necessary (unitize/shim gas system, NM trigger job, ream flash supp.,etc..) to make it a NM rifle. Best to have a trusted professional do this work. There are several on this board who do this work and are highly recommended. Stay tuned and you will learn. Expect a NM build from scratch to cost >2000 and up to around 3000.