You will be able to list your preferences for billets (you'll get a spreadsheet with 50-100 options), and will learn where you will be going around Week 11. There is no guarantee of getting your top choice billet or even a top-5 billet, nor are you even guaranteed a community (afloat, prevention, aviation, response). The Coast Guard generally tries to locate spouses together, but you have to be prepared for the idea that your spouse will be working in (e.g.) NYC, and you are sent to a cutter based out of Alameda. Further, if you do get an afloat billet, you may be out at sea for 2-3 months at a time for a period of 18 months to 2 years.
The military lifestyle is famously brutal on young marriages. You need to discuss these possibilities with your SO and make sure they're going to be okay with it/at least tolerate it, and know what to expect. If she isn't, you probably need to choose between AD and her, because it's really unfair to stumble into the service/marriage with the hope that everything will be fine.
EDIT> USCG OCS is very competitive for non-prior service applicants. You need to make sure you're building your resume now while you're in college if you want a realistic chance of getting in. Consider doing CSPI if your college qualifies, or volunteering with the aux if it doesn't.