All excellent suggestions so far.
I would also agree that analyzing your campaign performance through Google Analytics is your best course of action but I also know that it might be a bit intimidating.
Fortunately, there is one more way you can track your leads directly in your Unbounce leads list if you don’t want to deal with GA right now.
In essence, you would tag your links with what’s know as UTM parameters.
ex. If your URL is: example.com/landing_page/ when tagged with UTM parameters it becomes: example.com/landing_page/?utm_source=google&utm_medium=cpc&utm_campaign=bestkeyword
You can change the source, medium and campaign based on your needs (google, facebook, email, guest blog post, etc.).
There is a handy URL builder right here: https://support.google.com/analytics/answer/1033867?hl=en
After that, you just go into your landing page editor, select your form and add 3 hidden fields exactly matching: utm_source, utm_medium and utm_campaign.
Now, when you use these tagged URLs to lead visitors to your landing page, Unbounce will automatically fill the 3 hidden fields and with the appropriate content.
Later on, when you are reviewing your leads you’ll be able to see exactly where they came from.
There’s a bit more information on this method as well as what the other community members have recommended right here: http://documentation.unbounce.com/hc/en-us/articles/203509994