Welcome to the community!
I would look over the sticky threads under the Tutorials & Resources forums when it comes to setting up your first map:
https://tf2maps.net/forums/tutorials-resources.57/
As for my personal method, I usually begin by sketching out a a base plan of my map(basic geometry, crucial level features, etc.) on a piece of paper. This step is often overlooked by new mappers, but I can tell you from personal experience that this method helps tremendously, as having a set plan pretty much guides you through development. A common mistake many new mappers make is placing down a gigantic brush and putting stuff on top of it.
DO NOT DO THIS! This method of building 'kills' height variation, and makes for a really terrible area to play on; I know because I've created many maps with this method. I hope that this helps, and once again, welcome!