What legal measures do I have to take to start selling clothes I make?

As fashions change at a very fast pace, it is not economical to do registered trademarks for everything (only the business name & main logo & maybe slogan). A simple trade mark (TM) after the name of individual garments will do. For a license, yes you will need one for wholesale. For tags you can have the company name (if you want) but you are required to tell what it is made of & what percent, the size, and how to clean it.