I got started after watching
Pithy Bikes on YouTube. Beautifully made videos and bikes, and he shows his very first frame build on there so it's good for a beginner. He has a mini-mill and a mini-lathe, but you can manage without these. My recommendation is find an old bike frame to cut up and practice the welds (or however you're going to do the joints) until you're pretty confident. Start with 90 degree T joints but then move on to the angles and some of the tricker junctions you will get on a real frame. This is why an old frame is good-- you can keep cutting the seat-stays and chain-stays off and welding them back on again
You will probably also want to make some sort of basic jig. When it comes to designing the frame you can use scale drawings, BikeCad, or this completely free option:
https://codeberg.org/benc/framebuild.