I’m going to start a online business, what are some things I have to do to?

1) You don't have to create an LLC or corporation, but are strongly encouraged to do so, since doing so will help protect personal assets (e.g. your bank account; a house; vehicles) from business debts and obligations. Of the two, an LLC is generally the easier, simpler choice: you can set one up through your state's department of state. If you select "partnership" tax treatment, there will be no separate or double taxation to worry about; profits and losses will "pass through" to your own tax return.
2) If you do set up an LLC, you don't need to separately register to do business in your state; if you don't, and function as a sole proprietorship, you will have to register to do business--which can also be done through the department of state.
3) If you set up an LLC, go to the IRS website to get a taxpayer or employer identification number (EIN)--it's basically a "social security number" for an LLC--and you use it where or when you would otherwise use a social security number.
(By the way, if you do create an LLC, you'll need to come up with a name for it before you register.)
4) Set up a business ("operational") bank account for the business--you'll want to keep personal and business funds separate (which also enhances your legal protection if you have an LLC).
5) If you have a name for the business or your products or services which you think is potentially valuable, look into trademarking it; you can find information about and instruftions for trademarking on the U.S. Patent and Trademark Office (PTO) website.
6) Get the appropriate insurance to further help protect you and your  business; speak to a reputable insurance agent about this.