What should I present to a web design firm looking to invest in my idea?

Kudos to you for getting prepared to bring forth your business.

However, there are a few things you need to know before you embark on this journey.

You should be cautious about sharing your ideas with people you don't know. You might want to have them sign a non-disclosure agreement (NDA) before sharing your ideas with them. Many people will tell you that NDAs aren't worth the paper they're written on, but that's simply not true. If your NDA is put together in the right way, it may be completely actionable. It would be a good idea to see an attorney to assist you with this.

Also, not to knock your idea, but keep in mind that there are many people who have a promising idea for a website. If your idea is promising, you should want people to earn the right to work with you so part of your RFP (Request for Proposal) might be a request for them to submit a rough draft of how they think your site will look. This way you can effectively determine with which designer you most resonate.