Lots of Renpy tutorials online, it's easy to learn if you put in the time. For graphics, a LOT of games use Daz3D. That is another whole rabbit hole to go down and it is a ton of fun. I wish you luck, the world can use plenty more of these games.
Remember that Daz has hardware restrictions. You're going to need a good NVidia graphics card to get the most out of it. If that is price restrictive for you, Honey Select, Honey Select 2, and Koikatsu are also used, but have varying degrees of cartoonish appearances. These last three are faster making images, but need to be modded to work and look their best.
The best initial tutorial is to just install and try them out. You'll figure the basics out pretty quickly. Then ask questions and watch videos to do the things you haven't figured out.
Good luck, we need more good ones. Just look up things most people hate (grinding, etc) to know what to avoid - and once you start and put it out there, don't abandon.
Are you making one because you want to or because you think it will make money? Because if its for the second reason you'll burn out before turning profit and it will probably suck