Pamela,
Just so we're all on the same page, understand that no one is going to come to you and say, "Hey, I need a web video." That's no different from someone saying, "Hey, I need a DVD." Web videos are just a form of delivery, and any type of video can be delivered via the internet.
Since you mentioned corporate training videos, I agree with the advice CVille has offered. If you want to start a business, you're going to have to build some credibility, and to do that you're going to need a body of work that you can show off to possible clients. In addition to providing free work for non-profits, you can look into providing content for outlets such as MonkeySee.com and HowCast.com
Once you've have a portfolio that you're happy with, you should build yourself a website that has your work posted.
Then you start callin around and marketing yourself to companies. As you begin to gain clients, hopefully you can even more through word of mouth.