Manage finances
If you are looking for investments, get after them. You need to have an amount that will set up the business and keep it running for six months to a year. On the other hand, figure out where your living expenses will come from for the first year that you are in the business. It can be someone's support (parents, spouse, etc.) or money that you have saved up over years. You need to be secure on both of these fronts.
Register
Register your business with the proper authorities. Go to a legal department and inquire about the process. Consult a lawyer and an accountant. Take their help in setting up the business in a way that has no loopholes. Talk to them about the structure (sole proprietorship, corporation, etc.) of your business and get your due local and state licenses.
Go through legal formalities
Hire an attorney and take his help in legally setting up the business. Acquire specific business licenses if you need them. Acquire an EIN (employer identification number). Open up a business bank account. Inquire about every legal detail and go through all processes that help you go legit. You need to do these things by the book.
Make a team
You need to have the right team for your business to blast off. Make a team (it can be co-founders and a bunch of employees) and assign them responsibilities. Put them down in writing on a legal piece of document. You don't want disagreements inside the company after the wheels start turning. This is what ruins a business.
Set up shop
Now that you are registered, it time to set up shop - both online and offline. Set up a website and social media accounts. Rent an office space in a geographical location that will allow your business to grow. The location is critical as this is where you will meet your clients. All aspects of your 'shop' need to be attractive and convenient for consumers to reach you.
Start generating revenue
As soon as you are set, start generating revenue as soon as possible. Many businesses fail because they keep waiting for things to be 'perfect'. Be smart. Penetrate the market as soon as you are set and keep trying new strategies.
Learn and grow with the market instead of crafting that one perfect strategy that will make you rich overnight. Get a mentor and network 'with the right people' to help generate a steady revenue stream.