And once I have an inquiry, like I did yesterday, from a real person, I contact them right away. Many people like to talk on the phone along with using email. It's nice to hear a real human voice and it's nice to use email to get all of the details correct. I spoke with a (hopefully) potential client yesterday and we combined phone and email very nicely. I was able to get a very clear idea of what they were looking for, and my client got to hear my voice and the fact that I was very much interested in the project.
Don't be afraid to let people know that you are interested in their project, even if you don't really understand it all yet. That's the nature of creative projects. It's why they are both fun and a bit stressful. This potential client found me on a Google search, so they must have liked my work and wanted to know if I was available. I am!
And before you jump into the fun stuff, you have to do some business. The first thing that you need to do, after contacting the client and assuring them that you are ready, willing and able, is to write a proposal. The trick to writing a proposal is to get some help from a businesslike person. Proposals sound a bit formal, and they are. In the proposal you state how much you would charge. It's like an estimate, but really more like an audition. Chances are very good that your potential client is talking to other people, who may be just as talented as you. So be as specific as possible. Don't ever write "it depends". Of course it depends! But if you write something that open-ended you will sound like a flake. And if you don't write anything, you are proving that you are a flake. Who would want to work with you?
After listening carefully to the potential client, following up right away and writing a proposal, the best thing to do is to forget about it. While you may be anxious to start the project, they may be working hard on their end, meeting with people, etc. to make it happen. Don't bug them. If you haven't heard back for a couple of weeks, a gentle email inquiry is appropriate.
Responding to inquiries and writing proposals and estimates is part of doing business. It's no guarantee that you will be hired, but it's the closest thing I've ever found.