Lance Mindheim has written a terrific post called The Hidden Blessing of Constraints. I wish everybody in the hobby would read it, and heed it.
Lance concludes the post with several pieces of advice. To this, I would add two points (related to each other):
Select a prototype to model: Learning to model what is there will force you to challenge your abilities in a way that freelancing might not, because when freelancing it’s always possible to adapt one’s vision to one’s skills or available product.
Pick a manageable piece of railway to build, so that as you tackle the various skills required you see real progress on your layout.
There are plenty of examples on this blog that satisfy both of these points. If you’re new to this blog, I encourage you to go exploring…