I lived in an apartment for 6 months then moved to another apartment for the last 6 months. Which address do I use to tally up the total rental amount for the year?

Do not enter rent you paid in the Federal Rental property section.   If you erroneously started entering a rental property, you will need to delete it.   Search for rentals, use the Jump to rentals link and delete that rental property - the federal section is for landlords that rent out property they own.

If you pay rent on your primary residence, you might be able to claim a state tax credit.   There is no federal renter’s tax credit.    Most of these credits depend on the owner of your building paying property taxes. If they don’t pay these taxes, you may not qualify.   Select your state from this link.  If your state isn't listed, a renter's credit isn't offered.