I'm going to write this from the viewpoint that you are preparing for marriage and haven't yet had sex. I understand that not everybody that reads this blog is a virgin at marriage, but to make writing this easier I'll just aim toward that audience.I recommend Premarital Counseling for anybody wanting to get married. If nothing else, you will be given the opportunity to talk about things that will impact your relationship in a safe environment. This is a great environment to talk about sex and the basics of the sexual response cycle. Don't be afraid to ask questions or ask for resources such as books to help you become familiar with sex and help navigate your way through. It's also a great environment to talk about other real life issues like finances, role expectations, housing, values etc.
Be open and honest about your sexual history with your partner. If you've had sex before, they should know. Keeping secrets is never a good way to start off a lifetime of happiness. Your partner is the one person that should know everything about you. If they can't accept you for your past than you're probably better off without them. Also, don't guilt or shame your partner if they have had previous partners. Forgive them. Move on. Build your life together.
If one of you has a sexual history and the other doesn't, be aware that this has the potential to become a divider if not openly discussed. So make sure you talk about it if you feel self-conscious. Be kind and patient with each other.
If there is a history of sexual abuse, make sure your partner knows and is aware. Victims of sexual abuse may have flashbacks or react to a sexual situation differently than expected by their partner. It's important to talk about triggers and what is uncomfortable. Seek professional help if the sexual abuse is still affecting you and your relationship. As a witness to your partner's abuse, be patient, calm, loving and understanding. Never pressure your partner to do something if they are uncomfortable.
Discuss your sexual expectations. Frequency of sex, how arousal takes place, sexual activities, grooming etc. You may have to revamp this once you are married and actively having sex. In fact, you may revamp multiple times throughout your relationship. If you can start off your relationship talking about sex then it will be easier to talk about it later on.
Discuss expectations for birth control and child-bearing. If you choose to use birth control, decide together what method would be best and make it both of your responsibility.
If one or both of you experiences pain or some kind of dysfunction, seek help and guidance. Work through it together.
Be patient, loving and understanding.
Remember that sex is a divine gift from God. It is an opportunity for husband and wife to be united physically, emotionally and spiritually. Celebrate your unity with each other and enjoy expressing your love and commitment to each other.