hello! new to zapier. hoping you can help!

We have many events on eventbrite. Trying to push each new eventbrite attendee registration into Salesforce. We capture this in a custom “participant” object.  A very simple object with only three look up fields which ultimately creates a list of attendees on a related Case/Service object which captures the event itself. 1 event, many participants.

Participant record screen shot below. As you can see-  the Service number is the field that ties these two objects together.

All three are look up fields. And we want to search and connect to an existing contact if one exists, if not, make new.  I cannot figure out how to get my Zap to search for contact name and organization records - fields that we do have on the eventbrite registration- and in salesforce, either update those records or create a new contact and/or organization if neither exist in our system.  

thank you for any help!