To upload media, go to your profile, at the bottom of the page you will find the "Media" section.
Copy the URL of the media from YouTube, Flickr, Vimeo, etc. and paste it into the field, then click on "Import Media". You can also upload media from your computer by clicking on "Upload media".
After importing the media, a form will show up. From there you can edit the file properties like name, description and keywords.
If you want to insert that media into your community and assign tags to the different files, please follow this link: Can I create tags and assign them to media?