In the US at least any time you make income you are required to report it, regardless of how you got that money. Technically if you get $20 for your birthday you are supposed to report it for taxes.
Depending on your location and type of business, you may need any of the following:
a. Business License
b. D/B/A (Doing Business As)
c. Federal Employer Identification Number
d. Countless other things for a corporation or llc.
Ideally you should contact an accountant and lawyer when starting a business or at least read a lot about it so you have a decent amount of knowledge.
Re: starting a business questions everybody should ask...
when you start a reselling compnay, do you have to go somewhere to tell the government about it?
Unless you want to be audited by the IRS, you need to file for taxes. You'll also need to acquire at the very least an Assumed Name (commonly called a DBA - Doing Business As) unless you plan to use your real name as your company name. Incorporating (LLC,Corp.) requires you to file with the state.
Do you have to pay something for starting a business?
Most likely. Assumed Names cost a few bucks and registering as an LLC or Corp. generally costs 100-300 dollars, depending on your state.
This is of course assuming you're starting a business in the United States. There might be other costs involved (i.e. state sales tax) in which case ALWAYS check with a qualified state official and/or consult with a CPA/Lawyer.