The lead-ac id gel-cell battery is really resilient, you can treat it pretty badly and it will keep working.
Vlotage-wise, 14.4 is the fully charged voltage of a 12V unit.
For current, you should charge at a maximum rate of .2 times the current capacity.
So,
for a 12V, 7AH battery, you'd charge at 14.4v, 1.4A maximum;
for a 12V, 20AH battery, you'd charge at 14.4v, 4A maximum.
That said, the places that sell these batteries also sell chargers of all kinds: fast chargers, trickle chargers, you name it.
In Canada, we have a famous automotive/hardware store for such things. Here is what I use to charge a 12V 7AH battery. It's on sale too, act quickly. 
D