Newbie here!!!

Hi guys. I just came here today after being frustrated with a lot of apps/themes out there aren't exactly what I want, and started looking into doing it myself. Since its my first time here, i was wondering if you guys could give me some direction? What are some things that are a "must read" first? Can I make the theme on my computer FIRST and transfer to my device, or do i have to do it all in that tiny ass interface? What tools do i need if i do want to do it on my computer first? Must have apps, and software? and more "must reads? I'm on the mycolorscreen yt channel.. are there any others i need to watch? If this was answered somewhere I apologize, I tried the search function at the top.. but the keywords I was choosing were not helpful. I really appreciate any and all info you guys can help with.

Thank you so much,
Jayde