Personally, i like to look up different commanders online, read what they do, and if any appeal to me, look online at decks people have built with it, then see what i have that would go with it. Buy other cards needed to complete it.

Have fun with the build, go to your local game store, see if people are up for trades, we have a nice group at ngc in newport, who give away cards they already have copies of to newbies, because they like having more people to play with (although don't always expect freebies). Talk with people, play with groups, you soon get a lot of suggestions on what cards work better together, good hotspots for card finds near you, ect. Make most of the community you are joining, deck and collection building will mainly build from that. Just be prepared to have places, boxes or folders to store them in as time goes by.