First you will need to open Unity Hub and click on Projects in the left column.
Once you are looking at your projects, you will be able to click on the blue NEW button.
This opens a window allowing you to set up your new project. Type in the project name under Project Name. Make sure 3D is selected. The computer will select a default Location for you to store the project in on the computer's hard drive. What's already there is usually fine.
When you are ready to go, click the blue CREATE button. Unity will take a minute to load up your project. Then the editor will open and you will see a blank project. You're ready to create!