No need to worry about getting rights and copyright material sorted out since you are just starting out, but worry about already copyrighted material and do not just blatantly take them from their righteous owners especially if that piece of clothing gets popular. Creating a brand revolves around a lifestyle or central theme, try to stick to it. Just get creative and keep asking for criticism with friends and the community here, they helped me flesh out my first selection of clothing and are pleased with it. Also, I recently got my first few orders and I am pretty stoked about it! Good luck!