Charging battery using computer power supply?
Battery with nominal voltage 12V usually is charged with a little higher voltage. If this is lead-acid battery - that voltage should be 13.8 - 14.4V. 
Computer power supply is definetly not designed to be connected to battery. Partially discharged 12V battery can have voltage higher than 12V. If you connect it to computer power supply - you may feed power supply with energy. Power supply will "see" too high voltage on its output and will try to lower it to 12V. Depending on design of that power supply - bad things can happen.
You would need some circuit to limit current or some battery charging circuit anyway. Depending on battery type and size - you will probably damage battery or power supply. Battery can even explode, so never try charging when you can't measure current.
Many (if not all) computer power supply units can't work without some minimum load. You shall not power on computer power supply without load.


If you have no multimeter and no experience with electronics at all - I would recommend you to buy battery charger for that specyfic battery type (chemistry) and size.