Look around for a link or an icon that says RSS. Sometimes the journal homepage might itself be an RSS feed. Copy the URL in the address bar.
Alternatively, use a service like JournalTOCs to create a group of feeds. This may require creating a free account with the service, but it will ultimately save you lots of time searching for RSS feeds. Once you've created your group of feeds, you can export this list as an OPML file.
Adding Feeds to Zotero
To create a new feed, click the New Group or Feed icon then select New Feed.
If you have a URL to a single journal feed, select From URL, then paste in the URL. If you have an OPML file for a group of feeds, select From OPML, then upload your OPML file.
You'll now see your Feeds appear in the collections pane under My Library and Group Libraries (if you have previously created any group libraries).
You can browse your journals individually. Journal titles in bold have unread items; articles in bold have not been viewed. To add an article to your library, select in the middle pane and select Add to "My Library" in the top right. Note that the item metadata will usually require some tidying up, as the metadata coming from the RSS feed is frequently incomplete or improperly coded.