I was in a similar situation as you about 10 years ago-out of high school I decided that Horticulture was what I wanted to do with my life-I went to college for Horticulture and fast forward 10 years I am just noe getting around to working for myself instead of working for a company. It all started for me with 1 small job and mushroomed from there to a pretty steady gig. My niche is rehab of medium size landscapes for median income individuals, but I have done all kinds of jobs. My 2 cents are-
work for someone else first-even for just 1 spring-to see some of the ins and outs of the business-maybe even see how NOT to do things
Figure out what your area of expertise will be and pursue it-be THE expert in your area for a particular area of the business
Network with others in the industry-you would be surprised how often this can come back to help you
always continue to educate yourself-on plants ID, on paver construction, etc.
bottom line is-with a solid business plan, doing quality work, and just being there when you say you will be and doing what you tell your customer you will do, you will be sucessful.
also-check out www.groundtradesxchange.com-its a great resource for those in the green industry
email me if you have any more specific questions, maybe I can answer