I have personally invested in multiple startups.
If you have a decent sound business plan...quickest will be to be use one of the crowdfunding platform.
Micro ventures, Start Engine and Seed Invest are some of the platforms that I have used. Microventures will be at the top since they have had dropbox, docusign, lyft, airbnb, uber, slack, spotify, plantir, redgin and many other BIG names
You can decide to structure how you raise funds based on revenue sharing model or equity.