The hard part about freelancing is that more often than not, paying clients want to see your portfolio. Now when you are starting off, most of the time you will have little to nothing to show(or atleast I did :P).

I started off by helping out student organizations in my university and building websites for them. I then used those websites in my portfolio. Within the universities, some organizations caught wind of my work and asked me to build their sites for them. Soon, I had people tell me that their family/friend wants a website and I found myself building those sites. Then I started branching off into the community. I posted on local boards and got a couple more clients there. After this point, I started getting word of mouth referrals.

In a gist, you start with customers who will pay you little to nothing. Do a good job and advertise that work, you will attract higher paying customers. Having a few personal projects will also help, just make sure that they look good (remember "To the user, the interface is the product".

Good luck!