I want to buy a AR lower online and have it shipped to a FFL and build it into a rifle. How do I go about getting a serial number on it with the July 1st deadline looming? And is there any other things I should know? This is my first time building a AR and I want to make sure I don’t get screwed legally.

If the receiver is 100% complete it already has a serial number. The 80% receivers require a serial number

It sounds like this is what you are purchasing, correct?

Post a link

https://www.brownells.com/rifle-parts/receiver-parts/receivers/lower-receivers/ar-15-m16-a1-lower-receiver-prod107496.aspx

M16A1 lower

Let's see if this link works.  Hmm, I guess not.

Anyway, as stated above by Chaingun that's a 100% lower, it already has a serial number.  Contact your FFL to see if they'll do a transfer and how much they charge.  Just a reminder that your lower will have to be built in one of the CA compliant configurations. Either featureless or fixed mag. Neither of these configurations can be registered as assault weapons. However it will be registered as a long rifle at time of purchase.

Do NOT build it as a bullet button equipped rifle and try to register it as that would be illegal at this date.

Thanks for the info I really appreciate it. Would it be same if I were to buy a complete lower like this?

https://www.wbtguns.com/collections/receivers/products/bravo-company-complete-lower-m4-stock-california-legal

Yes, it cannot be equipped with a bullet button or registered as an assault weapon.  It must be built in a CA compliant configuration.  Upon purchase it will be registered as a long gun.