I spent about a year working for a mid-sized landscape company (30-40 employees) as a grunt and figured out I liked the irrigation stuff and got my custom applicators license. Spent a year as a service tech at the biggest irrigation company in my area. During that time I was doing chemical work for the earlier company on and off. Went to work for a year for a small landscape company with lots of clients and equipment but no employees, they needed a chem guy but did everything wrong.
Started my own sprinkler company, operated for 15 years.
Sold out to the biggest company in the state and spent three years there learning alot about the right way to do budgets, planning and produce consistent work. Huge learning experience.
Parted ways with them as they decided to stop doing all commercial work. I still did a little design and consulting work for them and they sent a fair bit of business my way. Started my own company.
Six years later I subcontract everything to guys I know and trust.
I deal with clients and billing and logistics (making sure dump truck loads of mulch show up to the right properties at the right time, etc.
It was humbling to realize work got done better and faster without me there looking over shoulders.
Your local supply house for chemicals and landscape supplies will be a great resource for info and a good place to stick up flyers to find a foreman who already knows what quality work looks like and how to get it done.
If you are good at the mowing side and enjoy that part (my brother loves to cut grass), dont mess up that line of business by doing too much yourself.
I'm sure you know who your good customers are, the ones who always pay on time and are good at telling you what they want and what they don't like when there is a problem.
Talk to a few of them about using you for all of their outdoor needs, they are predisposed to like you and will probably cut you a little slack on anything that isn't 100% that first season.
Drive through the nice part of town and when you see a building that looks great, stop and look.
Crisp bed edges make even a dormant landscape look manicured.
If you do use subs try to make sure you can pay them on time or early every time.
They will remember that when you are in a jam and need something last minute.
If a customer asks for something you aren't comfortable with, feed them with the long spoon, try to find a local expert to help you out before you quote a price or turn the work down.
Your local large university might have a co-operative extension service who can help with a lot of technical stuff.
The head grower and buyer at the Nursery will steer you right on all the plant related questions, they want you to succeed and buy more the next year
Also you may have a group such as SCORE (service core of retired engineers), who are retired businessmen looking to help out small businesses. It can be hit or miss, as to who they pair you with, but you can usually find guys who know alot about stuff that is a mystery to you.