First, you'll be prompted to create a business profile when clicking through the welcome screen. Once you create a business profile, you'll see some related templates to help you quickly share a post for your business.
On the Android and iOS apps, you can tap on the three lines in the upper left-hand corner to open the mobile menu, then "Add a business".
On the web app, you can do the same by clicking on the My Posts or My brand tabs and then selecting the down carrot next to your logo.