I expect Brian will PM you in detail, given his background, Sols. One thing that you should be able to get is some help from your local Small Business Bureau. They will supply you with some extensive checklists which you'd be sensible to look over first. I've put mine away somewhere at present, but off the top of my head just a few of the things to get absolutely clear are:-
Startup capital: how much have you got? Will you need a loan? (There's a section on getting business startup loans, etc.)
Partnerships - putting them onto a legal, business footing. Who owns what/what percentage of profit does each partner get/how much is each putting in? What are you paying yourselves?
Practical items: what tools and supplies are needed. Who will supply at best cost. Where will you keep the stuff. Insurances. Vehicles ditto. Clothing ditto.
Accountants.
Solicitors.
Insurances - yourselves re. accident/injury; your vehicle; your tools.
Potential clientele. How will you reach them: flyers, ads in local papers, Yellow Pages, etc. and the costs. What is your competition? Why would you be a better choice? Have you researched who, and where, they are?
One of the things the Saudi Oil Co. did/does was to help wannabe businesses with some of the checklists before setting-up. One was to do a three-year 'pre audit' for them, and the usual pattern of this is:
1st year: all capital expenditure. That's stuff that's likely to be around for a long time, one-off purchases such as a vehicle, a mower, strimmer, tools, wheelbarrow, hose, etc. That's the big bang to your finances, as well as finding the money for your solicitor to set up the necessary paperwork to register your business and your accountant to do your accounts. Advertizing/startup costs. Your salaries.
Difficult, if not impossible, to show a profit, due to the heavy capital expenditure.
2nd year: only replacement capital expenses (in case you lost or broke the stuff or it was stolen). Maintenance expenses, normal outgoings. Your salaries. Should show coverage of all outgoings with small profit.
3rd year: as per 2nd year, but as Capital is amortizing, you should show a better profit and continue to do so, provided you are offering a service that is wanted, and that has the potential to expand.
Do some research on your target clientele: see how often they have their gardens maintained, what they have done, what they pay.
Check out the costs of all of your start-up hardware, which will at the least be those shown above, plus other items such as protective gloves, pruning saws, secateurs, goggles, trash bags, spares for the strimmer, twines, tree wound sealant, fungicide, lawn treatments, and a general toolkit for making small repairs to broken plant supports, trellises, etc., especially if you're working for the elderly or less able-bodied at any time.
Write it all down, plus the costs of insurance for yourselves against injury, the goods and any vehicle you use for business, such as a van. Add the cost of hiring a solicitor for the first year, to formalize the business, plus registration fees, plus an accountant (unless you know how to do business accounts). Add in the salaries you both want to see out of the business. Compare the grand start-up total with how much income you think you can generate for the first year. See if you're close enough. Don't forget that major one-off costs will be spread over a few years, but that first year is a fair wedge to lay out.
You'll now have a rough idea of what your start-up, maintenance, and labour costs are versus income. If it looks good enough, but you need a loan, then you've got the figures done for your lender.