I'd recommend starting with writing. Just plain old script writing. Write descriptions of your characters too, not just what they look like, their backstory and mindset, likes, habits, hobbies, the stuff that makes them human.
Once you're happy you've got a good amount of script and you know you're not going to change any big details, at least at the start, and you're ready to start drawing: character sheets. Character sheets at least for the ones appearing in the first few episodes. You'll need these to 1) be able to concentrate on making a good character design that fits both the physical and backstory description of the character (don't go with the first thing that comes! Workshop it!) and 2) will be super useful to refer back to later so your character stays looking like your character. Remember to do at least one front and back view of each, it's a massive time saver.
THEN once you've got basic character sheets done, you can get scouting for references for locations to get a feel of the place these characters are going to be (you can even snap some reference pics around where you live if that works for your story, like coffee shops and parks and the like).
THEN storyboarding based off of your script. I recommend you structure your script cut into pages with individual panel descriptions within the page beforehand so you can really concentrate on making your storyboard fun to read, and so you know how much text'll be in each panel too. Then, go ham.
Good luck to ya