Start by reading this guide:
http://forums.androidcentral.com/op...-how-simplified-root-recovery-custom-rom.html
You can also scroll back a few days in this thread; a guy who was asking beginners-type questions was given a number of suggestions and links by several posters.
"Mirrors" = the same file, available at several different web addresses in case there is some problem with downloading from the primary source. You only need to download the ROM and the gapps files once; it doesn't matter if you get them from the main source or one of the mirrors.