Start simple and dream big
During my sophomore year of college, I tried to start a company I thought would be cool. It was a social site for young entrepreneurs that would let them post their business, create a customer database, keep track of their finances, print out advertising, and interact with other young entrepreneurs. It turned out to be too big of a project to undertake; I didn't have a clue what I was doing or the funding to make it happen.
If you have a big idea, simplify it. If the simple version goes well, start working toward the big vision.
Get a customer before you start your business
For the company that I tried to start in college, I spent weeks researching it, months writing a business plan for it, and years trying to start it. But I never asked my target market if they would use it and buy from it.
Before you start a business you need to validate your idea by getting a customer. For a service, you should have a paying customer. For a product, that customer is someone or a group of people who have told you, "Yes, if you created that product and it provides these benefits, I will buy it."
Find smart, talented people to help you
I've been a one-man show for pretty much my entire entrepreneurial career. But now I have a web marketing company and I subcontract a few of the services to other smart companies and talented individuals. Once you've decided to move forward with your business, you need people to help. At its heart, a business is simply a series of systems, you need people to run those systems.