There are a number of things you need to start making and selling clothing in the US. You should do these things as soon as you're serious about launching a company. I’m going to list them out below and since there are plenty of resources on each individual one, just do a google search. But make sure you have and know about all of these.
- Form a LLC or other legal entity.
- Make an operating agreement.
- Get an EIN (federal tax ID).
- Register as a sales tax vendor in your state of business. This will allow you to buy goods at wholesale (without sales tax) and also allow you to collect and submit sales tax to the state when a customer buys something from within your operating state.
- Register for an RN number, which you need to put on every garment you make and sell. It basically gives the government a way to track manufacturers and the products they make. You can also put your legal/operating name on labels in place of an RN.
- Get your website and emails locked down.
- Don’t worry about copyrights or trademarks for your name or products early on. It’s a waste of money.