100% agree with Ged.
Make a business plan that is realistic, account for everything from tea bags through marketing and insurance etc. Your business plan will change especially in the 1st couple of years. Keep it up to date and STAY on your figures, cash flow is king. A business plan is not just figures you must have operational plans, marketing plan and a plan for recruitment and delegation. Don't employ friends or family, at least for the 1st couple of years. Make capital expenditure decisions wisely, it's true that if you buy cheap you will buy twice. If you can, even at planning stage, structure your thinking as if business departments exist (marketing, sales, finance, legal, operations etc.) This will help you with your planning and will help you as your business grows.
Don't be greedy, be genuine and honest and be prepared to work 12 hours a day, 7 days a week, 365 days a year with little or no backup or support.
I use a strategy prompt for most business planning decisions. I visualise it as a triangle with these elements at each point: Statement of intent (what do I want to do and how do I need to do it), Foresight (What is happening in the future that will influence my business, why do I want to do this, what is influencing this decision, where will this take me in the future), Capability (what do I need to make this happen).
A final word of advice - If you want a day off from work in the next few years; if you don't want to deal with every single stupid BS problem; if you don't want to be absolutely responsible for absolutely everything and absolutely everyone involved in your work; if you want a holiday in the next few years .................. Work for someone else.
If you go for it enjoy the ride and try not to burn any bridges along the way.