When someone shares a Document or File, they have the option to notify the collaborators via email.
The email notification contains a link that will open up the shared Document for you.
Don't worry if you miss the email notification or if you delete it. You can always go right to the Documents and Files that are shared with you using the "Shared with Me" label in your Google Drive. Click the "Shared with Me" label on the left side of the Drive page.
All of the Documents and Files that are shared with you will appear in the list. You will also be able to see exactly who shared the Documents with you and when they shared it.