Mechanically, it's very easy to put together an AR-15 lower. You start with a stripped lower that can be obtained at your local gun store. This is the only part the the ATF considers a "firearm" and must be transfered to you via a licensed gun dealer. You can also buy them on-line and have your local gun dealer transfer it to you for a fee after it is shipped to him. You'll have to talk to your local FFL dealer about whether he will do this and for how much. $25-65 is a common transfer fee cost.
The stripped Lower:
Then you need a obtain a LPK (Lower Parts Kit) and a Stock kit to complete the lower half. Getting it on-line is the best way. Getting it shipped to California may be a problem, but the are many places that sell California compliant stock kits. I know nothing about bullet buttons, pistol grips fixed stocks and such that are required in California. The standard grip typically comes with the LPK.
Simple common tools are all you need, but a lower vice block and a AR-15 stock wrench will be needed to do the job.
The Completed AR-15 Lower Half:
The best way to finish your AR-15 for the beginner is to obtain a "Complete Upper", California compliance is common, and then just attach it to the lower half via the two pivot pins in the lower. This takes about 15 seconds. Then your rifle is ready to shoot. There are many instructional videos available on-line about putting together an AR-15.
The Complete Upper:
I built mine last year around Thanksgiving using all Rock River Arms parts and an Armalite stripped lower I got at my local gun store. The basic gun with iron sights and 1-30 round magazine came to $815 complete.
The worst thing you are going to have to deal with now is the pre-election panic going on right now. Wait times for parts currently are running 3-9 months depending on what you want. If you can find a fully assembled rifle at any of your local sources, I'd jump on it if you want an AR-15 anytime soon.
Good luck and enjoy your AR-15.