These are some wonderful answers here.
0. I would advise you to find mentors in the field you are working at. This is not obligative, but very helpful.
1. As any business searching for clients first objectify who are your ideal clients, what are their problems, what is your main focus that you are tatally great at and passionate to keep improving.
2. Linkedin, emails, yellowpages, directories, friends, recommendations of your surroundings. Everything works to find your first clients. As well as here you might start almost with lower prices to build up your portfolio and recommendations list.
3. Make sure your social profiles represent you as the professional, build your personal brand online. Great sources for that: facebook, personal blog, linkedin, youtube, anything that can be a social proof of who you are. People are buying from people.
4. Make sure all of your paperworks are proper for your country and your state, be open and clear with your business to build trust through your actions.

I will be glad to shortly speak to you and probably be helpful for your start. Please ping me, pretty sure we can exchange some experience:)
Good luck on your path!