What should I do if my visa is about to expire and I'm waiting for divorce decree?

It appears that you are a conditional resident alien and your conditional resident status will expire soon. First, you should file the I-751 with request to excuse the joint filing, the 2d proviso before your the deadline of your conditional residency. Your case is not likely to be denied but you might want to include any document to show that the divorce is filed and pending. CIS will send a Notice of Action which automatically extends your temporary residence for another one year. Although, a Request for Further Evidence "RFE" asking the divorce certificate may follow, but the more likely scenario is that the Service center will forward your file to the local CIS office where you will be scheduled for an in person appearance. There, you can provide the divorce and any and all documents to show that your marriage was entered into in good faith; that you are not responsible for the break up along with other documents that may be necessary. Leaving United States under these circumstances especially while your case is pending is not a wise course of action. I think you will ultimately find that hiring an attorney is the better decision for you to make

You will want to have an attorney review and assist you with the filing of the I-751. The likely timing of the issuance of the final divorce decree and the degree to which the two of you either do or do not get along at this point will need to be discussed.

You must file on time with or without the divorce decree. Just to make sure, you received temporary residency status not a visa, correct? In that case, then you must file. Once you have the interview, you may already have the divorce decree or just inform immigration of the status of the case. Immigration will not be so concerned with your divorce, but with proof that your marriage was valid ab-initio (when it started). I would recommend you hire the services of counsel to help you with this matter.

Yes. File the waiver with a copy of the divorce petition and proof of your marriage. Then USCIS will send out a request for additional evidence in a couple of months. By then, you should have the final divorce decree. Just send it in after the date the divorce takes effect to avoid any additional complications. And consult with an experienced immigration attorney.