Freelance writers have to be active in searching out writing assignments, and clients. Getting referrals from past clients is one way of getting new work and new clients.
Assignments will not come if you sit on your rump waiting for them. You will have to be proactive in searching, networking, and marketing your services as a freelance writer.
There are bidding sites to find work, also sites where you can post a resume and pray for the best. Usually these sites are in the very low paying end of the writing market. If the pay is too low, you should not take the pay. If writers will work for as cheap as some projects slated for, shame on them. Some of the pay is an insult to any writer, let alone an experienced writer. If you are an aspiring freelance writer, some of the pay per article is an insult to you as well.
If someone is looking to hire a freelance writer, clients should be willing to pay a decent rate of pay per hour, or per project. You will still have to find the work; it will not come to you.
Networking, referrals, visiting local businesses, the local Chamber of Commerce, good work, and not missing deadlines are the best ways of finding freelance writing jobs.
This means that you will have to sell yourself to businesses as well as writing. When first starting out, you may spend more time selling and marketing yourself, than you spend actually writing. Once assignments start coming in, you will still need to market yourself and hope for referrals from satisfied clients to get you more work. See if you can get testimonials from your clients to put on your marketing materials and web site. Testimonials will help you get writing work in the future.