At your exact age, I was married for about a year, had two step-daughters, an infant son, a "grown-up, responsible" job and a mortgage.
I wish I'd asked these questions!!
In addition to the
excellent lists above, I can add:
1. Spend time truly getting to know yourself now. Know who you are and what you want out of life. Maintain that knowing in every relationship and don't let anyone take it away from you.
2. Choose work that you LOVE. You'll spend a lot of time doing it so it should bring contentment.
3. When/if you have children, understand that they'll be around for a very short time in the grand scheme of things. Love them, love them, love them. It will seem like no time at all from the day you brought them home from the hospital until the day they leave for college, get married, join the Air Force...
4. Welcome your kids' friends. Be okay with the noise and the mess and the chaos and the empty pantry - it won't last forever (see #3). Besides, it's fun to be the cool mom's house where everyone wants to hang out, and you always know where they are and what they're doing.
5. Really live in your house. (This one goes along with #3 & 4). Own furniture you can put your feet on. Don't obsess over cleanliness or who ate all the food. Your house will stay spotless and your fridge will stay stocked for weeks at a time when the kids are gone.
6. Also related to child-raising, maintain your independent identity even though you're a mom. Participate in activities you love that don't have anything to do with anybody else's dentist appointments, music lessons, baseball games, or school activities. Keep your mind engaged in things that interest you. (I see a lot of moms completely lose their identity and at a total loss when the kids leave because they've been so wrapped up in the lives of their children that they don't have any idea who they are without the mom label).
7. Know that even when you get some jiggly parts, when the wrinkles and gray hairs appear, when gravity starts to wreak its havoc on your body, and you get new little aches now and then - in your mind you'll still feel like your 16 year old self sometimes. When this happens, it's ok to drive around with the top down, the wind in your hair and the Metallica cranked!
(We need a head-banger smiley!)