Hi Rosemary Mason. The two most common ways you can do campaign association is either through an external call called Associate Lead/Contact with Campaign or a module called Closed Loop Reporting.
If it is the first way you are doing it, you need to create the campaign in SFDC first and then grab the ID from the URL. You can then place this ID on a hidden field in the form or use the Update Form Contacts with Data rule in the processing. Either way, this field needs to be passed onto the contact record of the person submitting the form. Then (usually in your CRM sync program) there is a step that handles the campaign attribution. It will place contact who submitted the form into the campaign in SFDC that is linked to the SFDC Campaign ID on their record.
The closed loop reporting way handles all of this natively. When you create a campaign in Eloqua, it will fire a Create Campaign call (which you can see under Integration Reporting). This will create the campaign in SFDC and link it to the campaign in Eloqua (on the canvas). Now anyone that responds to this campaign (filled out form) will be linked to the campaign in SFDC through a system linkage (it does not use the external call).