How can I use a form from someone else?

It is not possible to do since you will need to have access to the owner, if you can contact the owner or you are working on the same organization he/she could add you as a sub-user account and allow you access to edit and view the submissions of that form, here is a guide about how to do it: http://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User. 

If you have the link for that form then you are able to clone it, only follow the steps on this guide to do it: http://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL. 

Hope this helps.