First off, Welcome to the Lawnsite community.
Now down to business. I am your age, 19 and in college. I got legal 2 years ago. In order to get legal you will have to talk with an accountant and a lawyer. You will need to start up a corporate account at a bank, and will have to get a tax ID number, or EIN number. For insurance, its all on how much the job requires, and how much you think you need to cover your a$$. I paid $1200 last year for $1,000,000 of coverage. This year I am paying about $2700, for the same amount, but I am covered for more things.
For pesticide apps you will need an applicator's liscence from your state which opens up a whole new bag of worms because of keeping records, getting recert credits, and a whole lot of other things.
In regards to taxes, you will have to pay corporate taxes, income taxes on the business, then income taxes on yourself once you get paid. Also you will have to pay taxes when you pay yourself; SS, Medicaid?, state tax, federal tax. Then the good one is sales tax, unless you are doing certain forms of work that can be considered capital improvement. When you charge sales tax; over by me it's 6.75%. Nice easy equation:
2 lawn cuts: $100.00
Sales tax : $ 6.75
Amount due : $106.75
You pay the money for taxes quarterly, yearly, monthly or whatever it works out to.
Also, SS when you pay yourself, whatever you take out of your paycheck, you must pay that out, and also match it from the business.
Now the benefits of being legal:
When you go to a bank for a loan for a vehicle, machinery, etc. you will have tax returns to show income.
If you do damage to somebody's house / building / person. They sue the business. They do not sue you. When you are legal your insurance company takes care of it. If you don't have insurance and you are not legal, you get sued they take whatever you own to get their money.

Now if you go over the money you are covered for by insurance, they can go for whatever assets the business owns.
What else? Um I am trying to think here, but its almost 11:00 PM here and my brain is starting to run low on steam.
Oh yeah, use search.