First, congrat's if you are engaged!
Age doesn't mean maturity, some people never grow up. And while being childlike is a good thing in a lot of ways (sense of wonder / fun / acceptance), being childish is something some people never overcome.
To be prepared for marriage takes 3 main things. Being the right sort of partner & appreciating the right partner for you after finding them, then renewing constantly the commitment to a covenant marriage where divorce is not an option but being miserable together isn't either so finding the best way to resolve conflicts & enjoy each other is kept a priority. And that is a life long project, to be prepared for marriage we have to adapt to all the curve balls life throws at you together as loving partnership plus all the crap that can pile on top of life's normal issues. Learn to apologize quickly in the way your partner needs & accept their apologies gracefully & quickly.
It is always a good thing to spend at least as much time preparing for the marriage itself as people do on a fancy wedding. There is a ton of great materials that are relatively inexpensive or even free to help. First make it a joint project to learn as much about relationships, marriage, passion & gender differences as possible. This will keep you a heads up on the normal stages that people, your sex life & marriages go through so you don't think it's the end when really, it's just the start of a new stage for you both.
Then make a commitment to spend quality time together (with & without sex) so a passionate marriage is nurtured & both feel cherished regularly. And that one can be hard if you don't make a concerted effort just life can be very time consuming!
Make good communication a constant habit. Again, tons of great stuff on that & keep practicing really LISTENING to each other which is quite different from just hearing out what the other has to say. Try to keep their perspective in mind, be as empathetic as possible & strive to find a win / win solution so you both have an environment where you can be happy with yourself, your partner & the marriage.
So take the best marriage prep / education class you can as soon as you can. Many are available & inexpensive. I highly recommend this one & all John Gottman's work:
Engaged Encounter Description
Or get a DVD program like this one:
Smalley Relationship Center Presents - The DNA of Relationships - DVD Series - 6 Weeks to Better Relationships!
Find great couples to be a support system to you both & pick friends that are a friend to the marriage too.
Realize that most people only unconsciously pick their partner daily but the more consciously you do that & put in the needed effort to protect your marriage & good connection to each other, the better for both & happier the union will be. The Family Man (Nick Cage) & 50 First Dates movies are good ones to watch & discuss that.
Take the online tests listed below now then on a regular basis (at least once a yr right around your anniversary if not more) so you can correct issues or a disconnect before too much time passes so resentments and/or a disconnect aren't mountains before properly addressed, get to them early as they can easier to deal with & fix when molehills instead.
Gottman’s Marriage Tips 101
Gottman's Marriage Tips 101
Since 1973, Dr. John Gottman has studied what he calls the "masters & disasters" of marriage. Ordinary people from the general public took part in long-term studies, & Dr. Gottman learned what makes marriages fail, what makes them succeed, & what can make marriages a source of great meaning. By examining partners' heart rates, facial expressions, & how they talk about their relationship to each other & to other people, Dr. Gottman is able to predict with more than 90% accuracy which couples will make it, & which will not. What advice does Dr. Gottman have to offer? Below are some of his top suggestions for how to keep your marriage strong. [SEE SITE]
====
Here are some other recourses to help get you started:
Online Exercises / Tests:
The Gottman Institute - Marriage- Relationship Quiz
Assessments - Smalley Relationship Center
Marriage Builders® Questionnaires
HumanMetrics - online relationships, personality and entrepreneur tests, personal solution center
The 10 Second Kiss(Ellen Kreidman)
The Five Love Languages: How to Express Heartfelt Commitment to Your Mate (Gary Chapman)
Getting the Love You Want: A Guide for Couples, 20th Anniversary Edition (Harville Hendrix)
The Seven Principles for Making Marriage Work: A Practical Guide from the Country's Foremost Relationship Expert (John Gottman)
Ten Lessons to Transform Your Marriage: America's Love Lab Experts Share Their Strategies for Strengthening Your Relationship (John Gottman)
The Truth About Love: The Highs, the Lows, & How You Can Make It Last Forever (Dr. Patricia Love)
The Five Languages of Apology: How to Experience Healing in All Your Relationships (Gary Chapman)
Amazon.com: Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships: David Schnarch: Books
Amazon.com: 7 Stages of Marriage: Laughter, Intimacy and Passion Today, Tomorrow, Forever: Rita M. DeMaria, Sari Harrar: Books
Amazon.com: The Marriage Benefit: The Surprising Rewards of Staying Together: Mark O'Connell: Books