Start with mockups to get a better, more concrete idea of what you're looking for aesthetically-wise.
After you have a more concrete idea of what you're looking for, I'd recommend to start prototyping core mechanics out.
Depending on your experience with programming, you may want to get strong understanding of basic OOP principles such as inheritance and polymorphism. (It'll make your life much easier, improve code quality, and avoid repetitive code.
If you're new to programming, or even just the framework/engine you're using, I'd recommend getting acclimated to your development environment by starting with basic mechanics, such as moving/basic animations for walking, as well as calculating the rotation angle to align your character to the mouse. After the very basics of moving the character are down, start with the next most basic thing you can, and slowly work your way up the ladder until you have a playable prototype.
