Have everything written down, and get legal advice. Whenever you're starting out a business or any monetizable adventure, always think in a futuristic perspective. Other than what the other members mentioned like personal marketing, brand identification, or product establishment–you'll need to manage your taxes, monitor your growth, and gauge your net worth.
Another important thing to remember: don't grab opportunities right away. This might sound like a negative thing to say, but listen for a minute. When you're growing, your presence will be more known to a wide variety of 'seekers'. You'll have brands reaching out to you, people trying to offer you all sorts of stuff and knocking at your door. Don't let them in... yet. Because before any partnership is formed, you need to master the art of contract negotiation. To do this, you need to know the quality of your work and your products, and you need to quantify it in its monetary equivalent. Never let anyone lowball you–unless they're a potential long-term asset.