Buy your first one assembled. Tinker with it changing out bits and pieces to learn how it all works. It's easier to disassemble someone else's finished work, reverse engineer, and make sure all the pieces that go back are supposed to go back, rather than start with a bunch of pieces and not be sure how or where. If it's in your price range, do what I did.. get the 692 magpul edition and tinker.
Edit: And I wouldn't travel all over the house to grab my AR, though I store it on the other side of a locking cabinet from my 1911, so I wouldn't leave it behind either in a home invasion situation.