If you plan on doing anything other than simple changes to existing code, you'll need to learn at least a little html and css. Lot's of tutorials on the net. I believe roll20 has endorsed
Codeacademy. Javascript is less of a requirement, but highly recommended/necessity to handle more advanced capabilities. That's not to say js can't be used on even the simplest sheet, but I believe it's a more "optional" skill set at the beginning. That said, some tasks just cannot be accomplished without implementing sheetworkers.
It sounds like your sheet will be based off of D&D 5e? Both the Shaped and OGL sheets would probably be considered advanced. You might try starting with the 5th Edition (Community Contributed) sheet by Actoba. github code:(
https://github.com/Roll20/roll20-character-sheets/...) FYI: you can grab any historical version of sheet from github. ie you can grab code from 3 years ago which will probably be a smaller, more simplified code base. Keep in mind that roll20 has released features that the older versions of sheets did not have access to at the time. Mainly the way repeating sections were handled.