Obviously it is easier for someone who has a year or two (or more) under their belt to look at their expenses and get an idea of what they are working with.
You need to set up a chart of accounts and do your very best to guesstimate what you will spend on all of these different categories... usually the things hardest to guess on are labor, subcontractors and cost of goods sold (fertilizer, seed, soil, mulch, etc.)
Numbers never lie. Everyone's situation is different. Everyone runs their business differently. Everyone has different expenses.