First off goto the state website http://www.iowa.gov/tax/business/business.html
and get setup to collect sales tax. It's pretty easy.
Mileage will be easier if you deduct it and not actual expense. You can make up your own mind about that. I use mileage, I'll bet most do it that way. Now the gas you burn in the equipment you need to know what grade it is and how much(gal.). ANYTHING that the business uses including floorspace in your house should be deducted. Don't cheat and make sure you get everything you can deducted. Don't hide cash or income because you'll want to show income to have borowing power down the road, and because you're not a POS deadbeat.
Insurance through NBA Schroeder is the cheapest in 100 miles. Get a liability policy and keep in mind that snow removal is a separate policy from mowing grass. I paid about 235$ a year my first year for landscaping liability.
Silly newbie, you get the money as soon as you can and get as much work as you can handle. Joking, but kinda not at the same time. I invoice monthly. Use the "Net 30 terms" language to sound really smart and fancy pants.
Welcome from IA.