It starts with a business plan; then you do an investor search, get an introduction, present a summary of the plan and, if the investors are interested, the plan itself. With that I just summarized in one sentence what people write whole blogs and whole books about. Which probably does very little good.
You might start with Getting Financed
, a chapter of one of my online books, which is free online. It's such a big topic, and that will at least get you started in your quest. Good luck with it.