How do I switch to couples counseling?

Since your question is about switching to couples counseling there is an assumption that you are in individual therapy which is great. Hopefully, you both have individual therapist.  Each person working on their own issues before beginning couples counseling can help the couples counseling be more effective.  

The couples counselor needs to be a separate person than the individual therapist so that one person in the couple doesn't feel at a disadvantage.  Or feel that the counselor is on the other person's side.  You will also be better prepared and know what your negotiable and non negotiable items for a relationship are for you.  For ReGain.com (part of BetterHelp, but focused on couples counseling) you will both need to sign up for services on the same profile to get couples therapy.  When choosing a therapist, go through the questions together if you can to choose what characteristics you want in a therapist to help you get a good fit.  If you are a good fit with the therapist you will both work harder. 

It is wise to seek therapy before getting married as it will help both of you be more aware of any biases or expectations that you have that may be or may not be realistic.  It also helps with understanding each other and separating the business of the relationship from the emotional part of the relationship.  You will also learn some useful communication tools that help each of you listen as well as how to talk to each other in an appropriate manner.  You will be taught skills to fight fairly so that a solution can be the end result of a conflict.  Always remember that when reasoning is high, emotions are low.  This is where conflict turns into a productive discussion.  When emotions are high and reasoning is low, the the conflict turns into things like yelling, shutting down, walking out, throwing things.  Basically behavior that doesn't need to be happening. 

I wish you the best in your endeavor, to keep this relationship intact, healthy, fun and a joy for each of you to be together.  Your children will also benefit from what you and your boyfriend learn in this process.