There are three ways to start sharing a file from Google Drive. They all open the same sharing window with the same sharing options:
From Google Drive, left-click the file that you want to share and click the silhouette of a person with the plus (+) symbol near the top right
From Google Drive, right-click the file that you want to share and click on "Share" in the menu that pops up
From an open Google Doc, Sheet, Slide, etc, click on the "Share" button near the top right of the page
All of these methods will open a pop-up window with options to share the file
This can also be done to share a folder, which will share the entire contents of the folder with whomever you choose
In the window that pops up, you can enter in the names or email addresses of people you want to share with
The pencil button opens a menu that allows you to set the permissions the person(s) has
Can edit - they have full permissions as if they created the file
Can comment - they can only add comments to the file
Can view - they can only look at the file
Click the "Done" button after entering who you want to share the file with. They will receive an email with a link to the file
Alternatively, you can click on the "Get shareable link" in the top right corner which will change the window to look like this
Doing so generates a link that can be copied by clicking on "Copy link". This link can be pasted in an email, a post on Google Classroom, another document, or any where that links can be shared.
Clicking on the middle menu allows you to set the permissions that someone has after they click the link
The permissions are the same as above:
Can edit - Full permissions as if they had created the document
Can comment - can only leave comments
Can view - can only look
Clicking on "More" brings up these options
These options will allow the linked document to be accessed by people outside of the organization.