In Drive, select the file or folder that you'd like to share and click "Share" in the toolbar. You can share via via email or a link. To share via email type a name or email address in the Invite people box. Then click "Viewer" and select the level of permissions you'd like to assign to the person you are sharing your file or folder with. You can also type a message if you'd like and click "Save". The person you shared your file or folder with will receive an email with a link to view it. To share a link, select "Anyone with the link and invited people" from the dropdown menu and then click "Copy link". You can paste this link into a text message, instant message, email or anyway you communicate.
Tip: you can add an expiration date or a password by clicking the gear icon in the top right corner.