Most of the images within VN's on this site are created either in Daz3D or Honey Select. Which one you use is largely dependent upon what graphical style you wish your VN to have. Honey Select is more cartoon like / manga style and games such as Harem Hotel use this software. Daz3D creates more realistic images, however also takes longer to render each image than what Honey Select does. Games such as Milfy City and my game use Daz software.
As far as getting up and going I can elaborate on Daz3D, as this is the software I am familiar with. The base program is free, however the assets used to create nice renders are for sale on the Daz store. There are however sites, including this one, where you can pirate the assets. For experimenting, I'm sure the content creators wouldn't mind too much that you pirate the assets. Should you create a VN and secure income from sites such as Patreon, you really need to be procuring these assets. To give some perspective, for a high quality VN you will likely need 200+ assets to create the content. Hence whilst the rendering engine is free, the cumulative cost of the assets is actually quite high.
Daz3D is not particularly difficult to use, however knowledge of lighting / photography is required to produce good images. There are a large quantity of tutorials to help you to learn Daz, including the ones you can find in the thread linked in my sig.