Hi Gary,
Sorry it took so long to respond.
The first thing I would do is to decide what locomotive you want to model.
Do some research of the prototype and what models of the loco are available.
Decide what model you would like as a starting point and the modifications that you would like to make.
Research how these modifications can be made.
You don't have to start with junkers. I do because I enjoy piecing things back together. If you feel you want to practice, sure, buy a few Tyco, Life/Like or Bachmann junkers to practice on, they can usually be found for a few bucks.
If you plan to detail and repaint an undecorated model will save time as you can skip the stripping process.
On the older locos, Cosmetically I try to improve the detail, Pilot detail, hand railings, grab irons and windows.
Mechanicaly I will upgrade the drive, regear and/or remotor or replace the whole chassis add better electrical pickup and better lighting.
New parts are becoming scarce. Thus the reason I look for junkers.
I also scratch-build or modify parts from other locos to fit.
I use detail parts from Detail associates, Details West, Cal-Scale and precision Scale.