Start by building your own web site, and make it look/work perfectly. You aren't going to get anyone to hire you if your own site looks shoddy! WordPress is probably your best bet as there are enough plugins to handle whatever your first few clients will want, and the community is there to help if you get in a bind. Joomla, Expression Engine and the like are a bit more complex to start. Steer clear of e-commerce as well, as there are too many things you need to be familiar with in order to build a secure, compliant site. The last thing you want is to have a site you built hacked, as you will be on the hook for any damages.

Start with friends/family if you can, at least they will be a bit forgiving if you screw something up - a paying client will not be. There are so many developers out there, the chances they will pick an inexperienced one over a guy with 10 years experience who only charges $5/hr are slim - and there are thousands of overseas developers who fit that bill.

You most likely won't be able to compete on the web or on sites like Upwork at first, so really try to get some referral business as that is what will really make you money down the road. As you build more sites and have a portfolio, you'll be able to get work easier.

Word Press Specific:
WP Hired (http://www.wphired.com/)
WordPress Jobs (http://jobs.wordpress.net/)
WPMU dev (http://premium.wpmudev.org/wordpress-development/)

General places to post:
Fiverr.com
Upwork.com
Craigslist.com
WorkingNotWorking.com

best of luck,

Definitely try Upwork! If you're in the USA, LinkedIn also has a freelancing network you can get involved with.

Check out some in-person networking events where start-ups hang out, and say you're offering inexpensive web design, but in exchange for a great testimonial.

And talk to other service providers, because they often have tight budgets when just starting out.

I can't speak to the freelancer's perspective, but I have hired people at Freelancer.com.

It will cost you a bit to respond to each lead but I would suggest looking at Thumbtack. (https://www.thumbtack.com/) You can create a profile for your freelance business and they will send you leads in your general category. The leads will be be in your geographical area and if the lead looks appropriate you can pay a small fee to bid on the project.

Most of the projects you will find on Thumbtack for web design are small projects with small budgets but doing a few would allow you get some experience and also give you an opportunity to develop a process for dealing with clients, changes, launches etc. so that you're prepared as you move to larger jobs.

Another great place to look if there aren't enough Thumbtack jobs in your area would be Upwork.com.