You have to get one. It's a longish kind of process but it's not too difficult once you get organized. Here are the basic steps. A more detailed guide will be available in the coming weeks but for now, use the message boards for advice if you get stuck.
First. Go to the USCG site and find the link for licenses and documents.
Here's the page to get started.
Second. Find the link for original documents and get the checklist of required items. Print everything and start a folder. It will contain some of the following: An application, a physical form, and the checklist itself. They want you to get some passport photos and some cash. There is money involved here. The site has a rate schedule.
Third. Find the link for REC (Regional Examination Center) locations. When you are sure you have your checklist filled out, you must take your application in person to the REC. Choose the one nearest you or one that you can easily find lodging if it is far away. Friends and relatives come in handy for this. Once you choose an REC, that will be where your records are kept.
Be patient and gather anything the Coast Guard asks for. Some individuals require more screening than others due to certain legal, medical, and service issues.
Eventually you'll get a Z-card with Ordinary Seaman/Wiper on it. Start the questionnaire again and go to work.