Seeking General Direction for Developing a tower defense 2D Game

Seeking general direction for developing a Tower Defense 2D Game We are 2 friends who are ambitious to create a tower defense 2d game.
We just have started to learn to make art (vector graphic design via adobe illustrator) and learn to manipulate the Unity editor.
We are not certain about our direction for developing the game because we are just newbies. Therefore, if somebody have experience about developing a fully-realized game, I hope you can share with us what we should learn and do.

Start with small steps… - Like this
How did I learn to make Games? (youtube.com)