changed battery now anti theft wont let me start truck and alarm keeps going off

Is this factory or 'aftermarket' alarm system? Try locking doors, now use the key to open drivers door and start engine.. IF it's factory, you have a key fob to disarm!!

Get a wire and connect each side of the wire.to the battery one in the positive and one in the negative leave it like that for about an hour the computer will reset and everything will start up good