It will depend upon your relationship with the investor. If the investor have trust upon you, then you may pitch anytime, however, if you don't know the investor for long, then you have to follow a process.
You may setup a call to discuss more.
Start a Kickstarter project.
This will give you early funding + you'll be able to negotiate far better terms for investor money.
You may even find, running multiple Kickstart projects provides better funding + you keep 100% ownership.
If the investor is a rich uncle who would do anything for you, pitch it today! :) Most people aren't that lucky so they prepare an elevator pitch (Think "Shark Tank") and business plan. Your pitch should describe your vision, the service, the potential value of your business, the amount of money you need and what you're offering early stage investors. Form an advisory committee to help you with your plan and find investors. While you work on your pitch and plan, start thinking about investors who might be interested. For example, investors/managers who understand the space and had success with a similar / related offering would give you a quick dose of reality about your service and your proposition to customers and investors. Angels with related experience and potential customers are good prospects too. One way to appeal to potential investors is with reviews / quotes from potential customers and former managers / owners of related / similar offerings. Test results with testimonials from the user/customer is the best proof. Let me know if you need help with your pitch, plan or start-up.
This is a great question and is often asked around the startup community. At a recent event, the three main ingredients the panelists concluded for fundraising are: 1) have a good story, 2) be prepared and 3) build relationships
You can start talking to investors now to build those relationships. Don't start by pitching to them, start by asking for advice. Be genuine.
A big milestone you want to get to BEFORE actually pitching is having a very tangible dollar amount that you need to get to the next milestone for your business. I call it the "rock-solid plan" when your financials line up with your marketing plans, product roadmap, and hiring plan.
Investors that do believe in you will want to know that the money they give you will get you to milestones like: Early Revenue, Product Market Fit, Cash Flow positive, etc.