What happens if my AOS application gets declined?

It really depends on what grounds for denial there would be. It's too general of a question.

If declined, you will be out of status and subject to removal proceedings. Why do believe it will be denied?

You may want to consult with an immigration attorney to discuss the process and your options prior to filing. Timing of your last arrival and the marriage will need to be discussed, as will any other reasons you may have concerns regarding a potential denial.

If your application for adjustment of status is denied by USCIS, your case may be referred to ICE for removal proceedings.

If your I-485 adjustment of status application is denied, DHS may issue and file an NTA, a decision, whose sole office is your deportation from United States. While it routinely approves most I-485 applications, especially where as in your case, the beneficiary is a US citizen relative, CIS is nonetheless with authority and can, where an adverse decision is warranted may deny an I-485 application. Noting that adjustment is a discretionary relief, your ability to challenge an AOS denial is limited. Although, you can renew your AOS application in a deportation proceeding, the outcome is never certain, especially where DHS' denial is supported by the facts on the record. For reasons best known to you, an adverse decision on a case yet to be filed is already being anticipated, I am therefore going to advise you to hire an experienced immigration attorney to whom you must freely and honestly and openly express your reasons (s) and concerns why you feel this case will be denied. If you fail to do so, you may have to deal with the consequences for years to come.

More likely than not you will be referred to an immigration judge for removal. Make sure you work with an attorney because it is extremely important your application is prepared correctly. My firm handles these cases in NY.