I would assume that you've already downloaded Unity and finished the registration process. If you did, then by launching the program, you will be welcomed by the Projects screen where all your future projects will be located. Right now it is empty, so let's add one to it by clicking on the New Project button at the top-right corner.

This will open a new window that will ask you to enter the name of your project and its location on the hard drive.

I'm going to skip the fancy naming and go with My Unity Project this time and leave the default Location. Make sure that you have the 3D option selected, which will default our project to the 3D space and click on the Create Project button to proceed.

Don't worry about importing packages at this point; we can always do that later. If you already have a project, creating a new one is done by navigating to File | New Project in the top menu.