There's a few ways to get started depending how your layout is setup. 3 Pieces you'll need (primarily) if your going for a standard setup with Protosound2 / Protosound3 engines is:
- MTH DCS TIU (MTH #50-1003)
- MTH DCS Remote / DCS WIU (If you want to hold a remote vs run the trains using the app on your tablet/phone)
- MTH TIU-TMCC Adapter (MTH #50-1032) (If you want to use your MTH remote to control the TMCC engines)
Setup depending on your current layout can be pretty straightforward. Your CAB-2 base connects to the TIU using the serial TMCC adapter for using your DCS remote to control TMCC engines (you can't do vice-versa) (if your using LCS, you'll need to get the SER2 and PDI cable to connect it), the common wire gets run to the common output of your TIU. Power can be run in a number of ways, personally I have my PH180 connected to the TIU Fixed #1 input, then wired from the TIU output to your track. If your looking for an in depth guide book there's the DCS Companion that was written by Barry.
I should note this is a very high level overview, there are many ways to get connected and running, I would recommend listing what you have setup now as far as legacy, as that can help you plan what your looking to move into as far as DCS.