My alternator went out so I replaced it now my battery not charging? It is a 1996 4 cyl Isuzu pickup

Always begin these problems by removing and fully charging the battery. While it's removed, check for any bad cells. If there are, replace the battery. Voltage across the terminals, with the engine off, should be 12.6+ volts. Voltage across the terminals, with the engine running, should be 14 to 15 volts (regulated voltage). If the regulated voltage is less than 14 volts, the alternator is bad. Finally, be sure to clean both ends of the "B" wire, which goes from the alternator to the battery, with abrasive cloth--then, apply dielectric grease.