It is not easy to become a successful illustrator. It takes a lot of work and perseverance to get to a level where you can support yourself from this kind of job. Here are some good tips for making it easier to get work and establish yourself as an illustrator.
Study your type of illustration. This doesn't necessarily mean that you have to go to college. Many successful illustrators are self-taught. Learn all you can about what you want to illustrate. If you want to illustrate children's books, for example, study children's books, join forums for children book illustrators, and read lots of how-to books. Above all, practice, practice, practice.
Get pro bono clients. A corporation or big book company may not let you create illustrations for it if you are just starting, but local businesses and self-published book writers probably will. You may have to work for free, but at least you are building a strong portfolio with samples done for actual people.
Advertise like a professional. Once you have several samples of your work you'll want them available for people to see. Get a website or join an artist portfolio website to display your samples. Also have a book portfolio, as well. Organize your samples so images that are similar in color or type are placed together.
Now it is time to find paying clients. Thanks to the Internet, this is easy. Bidding sites like Guru and Elance are great places to find jobs for illustrators.
Join an illustrator critique group to get real opinions about your skill.
Always have a contract for all work that you do.