The first thing you need to do is to set a wage for your services. In the start, you have to set your service charges a bit low. However, as you will gain experience, you can increase your wages. Once you have settled a fee for your services, you start looking for a job. Finding a job is not that difficult, as there are several businesses as well as citizens are looking for a full time gardener to look after their yard. It is recommended that you invest some money in business cards in order to get your name out in the market.
Visit local lawn services in search of work. Most of these companies only offer services for trimming grass. However, if you can assure any of these companies that you are seriously pursuing a career as a professional gardener, then it is likely that they will give you an idea about where you can find work. If you get work through their reference, make sure that you do that perfectly, as it will open up more opportunities for you.
Visit real estate offices in your local area, as you can also get an idea about which house needs its garden to be maintained.
If you are desperate finding a job, then you must also visit local banks, institutes and companies. There is always a chance that any of those buildings is actually looking for a full time gardener to look after their lawn, and you might end up having a good job.
Find out every local gardener in your area, and make good relations with them. They can also become one of the sources that will lead you in finding work.
Let all your friends and your social circle know that you have opted to pursue a professional gardener career. They can also help you in finding work initially.