While starting your career as a freelancer is going to be a little bit tough as you need to find clients related to your work domain. Why not start with the people whom you have known for a long time. Call them, email them or in simple word, contact them! This contact can be anybody - a friend, a colleague, previous clients from your job, some other freelancers etc. Tell them about your work domain, expertise, skills etc.
Talk to other freelancers related to your field. Be in contact with the best freelancers or you can say top-notch freelancers who are doing well in your domain. Get in contact with those expert freelancers and go smoothly with them as they will be helping you out in learning various techniques as well as they will be helping you in gaining some good clients.