Reddit mentions of The Inventor's Bible (Inventor's Bible: How to Market & License Your Brilliant Ideas)

I have been learning about this myself for the past few years. I have no legal background at all, so take this advice with a grain of salt.

WARNING: there are many scam invention development companies out there. Do your research on this!

Roger Brown is a successful inventor that chooses to show his ideas to companies that will sign NDAs [non disclosure agreements]. YMMV.

Another good resource with a different approach is The Inventor's Bible

You need to read about a few different approaches and decide what you feel comfortable with.

First thing you should do is do a google patent search yourself before you spend a dime. I heard a patent attorney say this database is better than using the US gov database. Then find a reputable patent searcher, patent agent, or patent attorney [listed in order of how much you will likely pay] to search for you, because they will do a much better [but probably never perfect] search for you. Try not to pay more than a few hundred dollars for a search.

Like Lutusp is saying, most times you will find there is something else out there that is the same as your idea, but you never really know until you search. To add to that, just because you get granted a patent does not at all mean you will make money. You need to learn a bit about how to analyze whether your idea can even possibly make money before you spend any.

Read, read, read reputable sources and talk to successful inventors in your area in person. You may have an inventors group near by. Search on google. Some are better attended than others so you may need to go to a larger city.

Hope this helps. Inform yourself well and learn to evaluate your idea before you spend.