Use the navigation at the top-left of your screen to navigate between the main sections of the app: dashboard, boards and organizations.
Note that the organizations section is only available if you're a member of a Trello organization.
Clicking on your avatar at the top-right will show a popup with several other options. This is how you access your account settings. You can also a quick link to the help pages, and the option to sign out of the app.
Learn more about:The dashboard
Working with boards
Working with organizations
Managing your account