It is a good idea to talk about the counseling prior to your appointment, to discuss expectations with your partner so that you are on the same page when it comes to finding the right counselor. Also, talking beforehand can save some of the footwork. If you and your partner are able to identify the "meat" of the problem, you can bring it forward and save the counselor some time and guesswork. If you and your partner have specific expectations or goals that indicate what you would like to see happen, write them down and bring those answers with you to the session. Couples who know where their problems lie, and have clear cut and realistic goals are more likely to succeed in marital counseling, and throughout their marriages, than those that do not know what they want, do not have realistic goals, and cannot identify the issues they face. Be sure you and your partner are both invested in staying together and doing the work necessary to pick up the pieces. If both partners are not all in, the counseling is unlikely to be effective.
Emotionally prepare yourself to dig deep and be vulnerable. Counseling is likely to unearth difficult things from your past. If you've never worked through our delt with these past things, it may be a very emotionally trying time at first to deal with these things. Eventually, though, you will be able to pursue healing in the long run.