Before doing commissions you should work on something that you can show them as evidence of your work. I made a hotel, but anything that shows off your work will be sufficient.
If you’re struggling, see if anyone would hire you for free! The easiest way to learn is by working on projects you aren’t so familiar with.
i started by building random stuff then going to groups I’m in and asking for an opportunity to work for them for free until i moved onto commissions, seems to have worked out somewhat
Work on something you like. It can be anything. I was building just random things I like before I started working for other people. The person looking for a builder is looking at your building skills - color choice, details, materials etc. So the theme isn’t really important from my experience at least.