You will need to get/borrow a voltage meter.
Set meter for 20vDC (20 volts Direct Current)
Engine and Key OFF
Connect Black probe to Negative battery terminal
Connect Red probe to Positive battery terminal
You should get 12.4v to 12.8v
At the battery you should now read 14.1v-14.8v
If not turn off key.
Find the alternator, on the back of the alternator will be a large connector with 1 or 2 larger wires connected to one terminal, this is called the "B+" terminal.
Put black meter probe on alternators case, put red probe on this terminal.
You should get 12.4-12.8v(battery voltage)
If not then you have blown fuses in the Power Box, they will be 50 or 60amp fuses, one for each wire on that terminal.
If you have 12.4-12.8v at the B+ terminal then alternators voltage regulator is probably bad