There are lots of things you may need in order to start a business. Since you are a minor this will most likely require your parents help. Please make sure you talk to you parents about your business so they can help you set up everything you will need.

Formal Structure--You may need to formalize your business by registering it with the secretary of state. This will vary depending on where you live. Also, depending on what you are doing you (and your parents) may want to have a formal legal structure set up such as an LLC or a Corporation.

Bank Account--When you first start a business you may only take cash and may not need a bank account. But as you grow your business you may find that a bank account is necessary to take other forms of payments (such as checks, credit cards, venmo, paypal, etc). Since you are a minor an adult will have to be on the bank account.

Website--You can register a domain name through a variety of websites (this site was registered on namecheap.com). You will have to pay a yearly fee for your website name. In addition there are a variety of website that will help you design your site and host it for a monthly fee. These fees vary depending on what you are using it for. Some sites help you sell products on your site directly. These also offer options for taking credit card payments.

Selling items online--There are lots of sites that allow you to sell products online. You will want to check around for the best option for your product. You may want to sell directly from your website. But it also may be easier to sell on a site such as etsy, ebay, etc. These sites charge a percentage or fee but are searchable and may drive more traffic to your items.

Taxes--Most of the time when you are selling an item as a business you should be charging sales tax (depending on the state you live in). If you are selling products on your website or in person you will need to be charging sales tax based on your state laws. Sometimes when selling directly through a third party site they collect sales tax on your behalf.

Shipping--if you are selling online you will need to find the right way to pack and ship your items. You can use USPS, UPS, or FedEx to ship items. Many times if you're a business the sends out a lot of products you can get discounted rates.