Next step is to pull the battery, take it to your local AutoZone/O'reilly/etc and have it tested. Do not remove the battery with the engine running! If it tests good ask to have it fully charged, if bad than buy a new battery and ask to have it fully charged.
Now that you have a known working, fully charged battery install it.
Check the belt that drives the alternator, is it appropriately tight? We want to make sure the alternator is turning.
Start the engine and with everything electrical off, measure the voltage at the battery with the engine at 1,500 rpm.
Next turn on all lights and the ac, what is the voltage at 1,500 rpm?