Look back at the statement you just added. Below it, there's a line that says "0 references". We need to fix that.
Locate a source that supports your statement. Now click on the "+ add reference" link to add a reference. Wikidata will make some suggestions in the property box. Right now we'll look at some of the common ones
- reference URL (P854) – use this if you want to use a URL. Just paste the URL into the value, and click save.
- stated in (P248) – use this if you want to use a database, a book, a journal article, a newspaper article, or something similar. To use "stated in", the source needs to exist as a Wikidata item. If it doesn't, you'll have to create a new item.
You can always learn more about sources by visiting Help:Sources on Wikidata.