If you're starting a company, get ready to start selling. Whether you realize it or not, selling will be your #1 job for quite a while. You may think, "not to worry, I'll hire salespeople!", but that's not the type of selling you'll be involved with in the beginning.
The first type of selling will be pitching. It will probably start with pitching to potential partners/founders. If you can't convince someone to join you, you need to work on your idea, your pitch, or both. Next you'll pitch to investors. You're trying to sell them on your idea and on yourself. They need to buy into both in order to hand over their money. Next, you'll probably pitch to employees. Why would they want to work with and for you? After all that pitching, you'll finally be in a position to sell to customers.
Here's the catch: Your sales pitches for all of the above mentioned audiences have to be done before you start with the first one. After all, your partners/founders will want to know how you'll sell the idea to investors, employees, and customers. Your investors want to know how you'll sell to employees and customers. Your employees will want to know how to sell to customers. After you have your various pitches ready, there's one more to work on: the elevator pitch. This is a 30 - 60 second explanation of your idea designed to get people excited to hear more.
I recently had the opportunity to work on my elevator pitch -- literally. I had announced my departure from my current employer and lots of people wanted to know "where I was going". My office was on the 23rd floor of our building, so I had between 45 and 60 seconds to tell my story. I finally got it down to about 30 seconds and found myself getting lots of follow-up questions, even after the doors opened. That's one of the signs of a successful pitch, the last thing you want is for someone to say, "sounds nice", and then stand there silently.
If you're working on a pitch I recommend being able to answer the question, "What makes you different?" That's the #1 question I got after telling people that I was starting a new pizza chain. After you can answer that question, get it down to a 30 second answer. As a final step, throw in one example of your differentiator in use . . . the lights come on after your audience can picture your idea in action.