If you want to share a file you login on the web portal of your ownCloud.
Find the file you want to share, and click on the link "Share" (on the right). Now you can fill in the name(s) of other ownCloud user(s). Or you can click on "Share link", which wil display a link which you can make available to to person(s) you want to share this file with. Optionally you can provide a password and/or an expiration date.