Need a vent and trap???

Just ripped out old tub and replacing with a shower. Drain needs to be relocated in this old house. Coming from existing drain, what do I need from old pipe to new drain??? Vent, trap, then a drain? or is the vent after the trap. Shouldn't there already be a vent? Not sure if and/or where it is.

drain

Without knowing more about your existing system, there is no way we can tell if you have a vent, if it is adequate for the revision, or if you need a new vent. In any case a vent goes near the trap, between the trap and the connection to the drain system.

This is tough without more details. However, the first problem you might encouter is that a shower is supposed to use a 2" trap and trap arm (the pipe that goes from the trap to the drain stack). Most tubs will use a 1.5" trap and arm. The 1.5" trap will likely work ok, but if you intend on getting this work inspected, the inspector will likely be looking for a 2" trap. If you want to go with the proper 2" trap, then you cannot simply extend from the existing trap arm to you new shower drain. Instead, you will likely have to cut in a new wye or sanitary tee into your stack. Anyway, there is no sense getting too far ahead here. If you can post a photo of what you are starting with, you might get more feedback.