The most useful training tool: books - what it means to actually "train" and how to structure your training. I prefer the "Cyclist's Training Bible" by Friel but there are many. I don't know how detailed the "Time Crunched" series gets on the macro training structure, but it's important to understand what "Base" vs "Build" is and why you do them.
Get a Garmin/whatever ASAP - without that the heart rate strap is useless.
Upload the data to Strava/whatever so you can track training stress.
The biggest thing is to be consistent - ride as much as you can, and you'll go far!
Yes power would be great - way better than cadence or HR - but expensive. Save that for later.