1. Register your name- go to your local government agency and enquire there about registering a name
2. Checks - check out some local banks in your area and see about setting up a checking account for a business. They require that your business name be registered with the state. Depending on your bank, you will either be charged a monthly fee for that account, charged a fee per check, or both. IF you don't mind having the customers write the checks out to you, setup 2 checking accounts (one personal, and one XYZ Lawncare) and just use the checking account as your bank account. You will need a co-signer for your account, and they will then have access to your funds. Usually just get your parent to sign. They need this incase you write checks and overdraft and decide to dissappear (they don't want to loose money- and legally they can't pursue or attempt to recover money from a minor/ nonadult).
3. Website?
www.directnic.com is the cheapest server i know of. $15 register domain a year, $15 to host advertisement free site. Throw a few pictures up there, contact info, and experience and other stuff about your business. It makes you more credible than just some 17 year old with a mower- should help getting commercial contracts.
4. Business cards- put a link to your website along with regular info and therefore they can "learn more about" and "get connected" (key advertisement phrases) about your business on their own time in their own convenience.