So, what you've been hearing so far is that there is a lot to it that goes beyond just selling. Taking a sales training course is a good idea but that presumes the elements that lead up to selling are in place. You want to go into business so start with a business course. Understand, you need to be a business man first and foremost. Rick's suggestion of the E-myth book is a great starting point to understanding that.
What exactly is your service? Why is it needed? What is the unfulfilled demand in the market that makes you believe there is room for your company? What will be your competitive advantage? What is the value for your clients? Who exactly are your clients vs. general clients for a tree business? Where in the market will you fit? Therein is the beginning of your business plan of which part will be the marketing plan.
As the owner of the business regardless of where you see yourself fitting in to the business, sales, production, operations, finance, etc..., you are still the one who needs to be able to articulate your vision, value and mission then train and manage your staff to embrace and follow it. This will also play into selling.
Selling is the process of connecting your services to the right clients. Done right, there is no sleaze or sliminess. The prospect becomes a satisfied client who feels they will receive value for their money. Doing this right, requires the salesperson to know their products and services, understands their target market and communicates effectively to the prospective client in a manner that is best suited to that individual.
On that subject, remember people process information differently. Your job is to figure out quickly how the individual you are addressing does this. Best tool for this are taking in the visual cues and effective listening. As mentioned above by bhlrp1962, the way they dress, the cars they drive, how they maintain things, their interests, their professions are all cues to what they value and how they will process information. Engineers think differently than do doctors and artists, and accountants or the small business owner. Listen to the language they use; do they differentiate or associate things? You'll need to tailor your approach to suit how they see things and gain understanding to best communicate your services.
One last thing, estimating is not selling. If all you do is drop an estimate without explanation beyond the basics specs then they are buying you are not selling.
Good luck!