Kung medyo newbie ka pa, just bring it to a radiator repair shop, and let them worry about finding the leak. But if you want to go the DIY route, observe the engine bay while the car's engine is running. Have someone rev the engine, kasi usually sisirit yung tubig kung may leak. Check mo rin yung hood mo kung may water marks. Dried coolant will leave a tell-tale mark. Usual suspects naman are hoses (main and bypass) and a leaky water pump.

Mura lang ang top overhaul ng radiator, mga P450-500. Rad shops naman will tell you if its no longer repairable, and will advise you na lang to buy a brand new one.