Start small … work on something more simple first. You could consider contributing to an existing plugin you like the look of, but lacks a feature you would like it to have, submit a PR.
Your main goal is something that can motivate you to get more and more competent but don’t make the mistake of taking on something too big, getting stuck and giving up. Perhaps pick on one small feature of your final goal … and build that … then add another. Rome wasn’t built in a day!
There are a few guides on getting started people have written. Here’s my aggregation.