cvraghu

I've a Nissan 2000 Altima. I found that my coolant is leaking on top of radiator. There is a crack or small hole. I can see white color over my radiator and hood.When i press the hose the coolant leaks out of the crack. The coolant level drops soon. I topped of on monday evening and saturday morning it is well below in the coolant bottle. Is this fine ? I'm very new to car repairs. Please let me know

1. How soon i shd replace the radiator? Is it safe to drive?
2. Is it fine to order it from any online site like radiator express?
3. I read in forums that it might one row core or two row core. Which one should i order?
4. Also whether plastic or metal?

Thanks in advance.