How do I connect a 1 1/4" tailpiece to a 1 1/2" drain line? My present connection is leaking at the threaded female nut.Tried to tighten as much as possible, but to no avail. This is pvc, and not metal. Looked at the big box store, but the clerk there knew even less than I do. Any help would be appreciated!

There is a specific tapered washer that goes under the slip nut that does the job.

Epmiller is correct. Look in a 1 1/2 trap and you should see two nylon seals. One for 1 1/4" and one for 1 1/2" tail pipe.