It’s all about luck. I did one simple website for a friend of mine, who’s a photographer. I’ve been overrun since then. :-)
Start doing some sites for people you like, without charging. It even makes you feel nice to help ;-) Then if you are lucky, he or she will have a bunch of friends in need of service, and after you have made sites, relatively cheap, for all of those, you have a pretty solid portfolio.
I haven’t gotten to make my own portfolio yet – I just reference the sites I’ve made in emails to those who write me. (I guess that’s uncommon for freelance designers – at least the start-up ones – my clients are coming to me, I’m not doing much (if anything) to get them)
So – use your network would be the only advice I could give…