I'm currently in the process of interviewing candidates for seasonal position in my Costco. Everyone must start on Costco.com. At the bottom of the page in the left column in the center, click on the jobs link. Fill out the app. Please include ALL work experience, references, and other pertinent information.
Once you've filled out the app, take a cover letter or resume to the building you applied to and speak with an AGM. Introduce yourself, briefly surreal with them about your interest, and then ask if there are any positions to be filled. I personally love to see candidates take the initiative to come in after an app is submitted. I usually pull their app immediately and, assuming I have time, I'll interview then on the spot.
Most hiring at Costco is going to be a part time seasonal position. This is going to be different at every building. Seasonal is the easiest way to get your foot in the door. After season, we might keep 5 or 6 of the 50 we hire.
In the last 3 years, I've conducted over 300 interviews. I've hired about 135 of those other people for seasonal position. About 40 have become perminate.