I have a low understanding of how to give my players information on Forbidden lores in most areas. Is there a book I can read or do you mind telling me about them? I know some about Orks, Eldar, the Warp, and the Webway.

You can use the lexicanum warhammer 40k wiki. Also if you really want to mess with your players you can always just make things up and run it as canon to keep it interesting to the players who know the universe but since you changed the canon to make it your own than that messes up with their knowledge. I just do that bc my players love the 40k universe so they know alot about it but I change a few things about it to keep them on their toes.

I would consider doing that if I knew alot of the lore myself. I have a vague gist of most of the lores due to their descriptors but thats alot of "new" lore.

Wiki crawling is usually my primary resource. If you have specific questions asking on this forum is also helpful!

Remember it's your campaign, so if you're ever unsure go with something, and then change it later and say your source in question was wrong. That's the Black Library way!

I basically use Forbidden Lore as a means of telling your players horrible things they wish they didn't know. The kind of things that will tempt the money-grabbing Explorers into doing something stupid that the average Imperial wouldn't want or know to do.

I have a mix of players - half know the setting very well and i often have to tell them to stop assuming they know what something is and actually roll their Lores. The other half don't know the setting so they use their Lores a lot to ask me questions.