If you're having a traditional Jewish wedding, you'll need a Rabbi to perform the ceremony. Finding a Rabbi can be a challenge when you don't already belong to a synagogue.
First you'll want to become familiar with the synagogues in your area in order to identify Rabbis that might be available. Local Jewish organizations will likely have lists of all the synagogues in the area. For example, in Chicago, the Jewish United Fund's Guide To Jewish Living lists all the synagogues categorized by denomination. Keep in mind, that if you are having an interfaith wedding, you will likely be limited to Reform Rabbis.
Start contacting synagogues and ask what the rabbi's policy is regarding performing ceremonies for non-members. There are a variety of possibilities. Some Rabbis will not perform weddings for non-members. Others will simply ask for a donation to the synagogue. And others might require that you pay the membership fee, effectively making you a member for a year. If the synagogue requires you to pay the membership fee, ask if they offer discounted membership fees for young couples.
Once you find potential Rabbis, set up appointments with them all to see if you are a good fit for them. You'll want to inquire about any requirements or restrictions that the Rabbi has for the ceremony, what he or she typically includes in the ceremony, if you can customize the ceremony, how far he or she will travel, and whether you'll have the traditional Jewish marriage counseling leading up to the wedding. Every Rabbi is different, so meeting with them ahead of time will help you find the perfect match for the wedding that you envision.