I would cut the elbow and coupler off, leaving just a stub sticking out. The P trap, sometimes called a "J" or "U" trap, comes down from the sink then curves back up toward the sink (like a "U" ), then turns 90 degrees and goes into that stub coming out of the wall. The actual connection to the drain requires a threaded adapter glued on the PVC pipe with a slip joint connector screwed to that. All of the connections from the sink to the drain are made with slip joints connections that are just hand tightened. The reason we cut that elbow off is connecting downward into that would create what is called an illegal S trap. That's a P trap with an extra curve. Any decent hardware store can assist you in getting the right parts for the p trap. They usually come as a packaged set of pieces with everything necessary.