From what I know, if you buy new and lease, you can write off the lease. If you finance, you can write off a percentage only, along with mileage. Keep all your receipts for tools and the trailer. You have to be careful that you don't write off more than what you make, or you might send off some red flags. In Ontario at least, you can forward unclaimed expenses, such as tools, the following year once you build up more profit.
I've always done my own accounting (software can be deducted!). Anything business related, is considered an expense. If you grossed $50K, spent $25K on the business between equipment, office supplies, insurance, additional staff, etc. Then take the $50K you grossed, minus your taxes on that, subtract the $25K and you'll have an estimated net profit. Now, when you do your taxes you might be able to save some based on what your expenses are. If you paid tax on the truck, trailer for example you can deduct that (again, Ontario here). For insurance, there is no tax on it here, so you can't deduct that, but you can still claim the total amount as a business expense.
I didn't mean for it to sound confusing, but when you go to register your business name, a small business center (at city hall, or another gov building) can supply you with a TON of info on operating a small business. It's a LOT of reading and researching and making phone calls, but it's worth while in the end as it'll save yourself some dough.
Other guys will chime in here with their ideas, suggestions and even facts. More so because I know Canada and US regs are so different.