How do I add a P-Trap to a copper drain?
What size is that copper?  You can get plastic p-trap kits in 1-1/2" and 1-1/4", and these can be easily connected to your copper via slip ring compression fittings.  Basically they consist of a nut with a cone-shaped plastic washer, which when compressed by the nut squeezes against the (copper) pipe to create a seal.  Usually these are only on the sink side of things and the wall side has glued-on plastic but you can use the compression fittings on both ends.

Here is a 1-1/2" Plastic p-trap

If you want to solder, a better solution than using a compression ring on the wall side would be to sweat on a copper x female adapter, followed by a male PVC adapter and a PVC trap adapter.  Better yet if you can find one, just use a brass copper trap adapter like the following:



A less desirable but acceptable solution would be to use a fernco fitting to go from the copper to PVC, and then you can avoid soldering.