I cannot comment on anything California, so my suggestion assumes you know what is legal and not
Go through the AR15 photo threads and identify a general set-up you like.
example: Here is GhostRing's pic (first one in the thread)
I'd then list the parts here I do not have to make mine like this
I would then shop each part for the best source for price availability and service
Buy them
Put them on, or in the case of barrel work I may have a certified armorer do it.
Specifically, I would ditch the bull barrel idea. AR15's are popular in part due to their light design. Don't load up unnecessary weight for one tenth of a percent increase in accuracy
Get a good collapsible stock, Knights RAS rail set up, a Troy BUIS, and Aimpoint or EOTech, and a proper M4 style barrel and you are livin large.
In summary, plan it out, pace yourself by budget or design, and enjoy the process
They are really a blast once you get a set-up and design you like
Good luck and remember to post the pix!!