As you say you realise you don't really know how loco's work, you could do worse than buying yourself a copy of Build Your Own Steam Locomotive "Sweet Pea" or Simple Model Locomotive Building: "Tich" and "build one in your head" over Christmas.
There are others but these are he simplest loco's so you should get an idea of where everything goes, what it all does and some notion of what you will need to make. After this you can start to choose what you would really like to make and perhapse get going with a Stuart Turner kit for a simple vertical engine just to get some practice.
I would ask myself the following.
1. What Guage do I build to and is that possible on my machines.
2. Do I want to build the boiler or get one made.
3. How much time do I have (a) to spend each week and (b) in total.
4. Do I predominantly want a loco to use or is my interest more on the build.
5. Do I want a popular loco where there is a lot of support regarding literature, castings, laser cut parts or an I happy to go it alone with a rarer design.
What ever you do remember that the most important thing to produce is enjoyment. So the very best of luck.
regards Martin
Edited By Martin Kyte on 15/12/2017 14:28:03