As an almost total beginner… could anyone talk me through this roughly?
Sure - starting with the advice fromdblue:
Open Renoise’s Preferences from the Edit menu (or Ctrl + Comma) and go to the MIDI section. You should be able to choose your Trigger Finger there as the master controller.
You can also set things up on a per-instrument level from each instrument’s MIDI tab.
Yes, you can set the TFP to play a particular instrument on a specific track.
By default, however, the controller is, ehm…controlling… the selected instrument on the selected track.
This is unlike most other DAWs where you have a strict relationship between an instrument and it’s track. Usually you have a simple scenario in which a drum machine might live in the third track. and is controlled by MIDI device X.
Renoise is more free-form in this regard - any instrument can (unlesss it has been “hard-linked” to a track, but more about that later) output audio on any track.
What you really need to think about is how you would like to play.
Would you always play a particular instrument, or would you like to switch between instruments as you play?
Would you like the instrument to output audio to a particular track, or the selected track?
By default, we control the selected instrument, which is playing on the selected track. But by going into the Renoise instrumentMIDI tab and selecting atrack allow you to make things more “tightly controlled”
You can even associate the TFP with multiple instruments, and specify a range fo the keyboard (a.k.a. keyboard splits).
And if you are planning to always do things in such a particular way, you can save yourself some time by saving that song as a template. Perhaps a bunch of empty instrument with all the assignments set up in advance?
Other options involve tools - the rabbit hole goes quite deep with this new one, for example. Generally speaking, tools can offer a “controller-centric” approach where you are not specifying track routings from the instruments but rather, from the controller itself.
So yeah, quite a lot of different options 
Of course, how you really prefer to do things boil down to personal preference, but before you can even arrive there you would need to have a mental map of what is possible and what isn’t.
Hope this helps!
