Become a man before you become a husband and a father. Break yourself in character-wise by major challenges that push you beyond your limits (as parenthood will do). So Peace Corp, military, overseas charitable work, etc. I mean blood, sweat and tears. This won't come naturally and most people need coercion to do it, so join something that makes you do it if you have to.
This way when you do decide to marry and have kids you'll be that much more certain about it, and that much more prepared for the crucible. You will already know what you are made of and capable of owning these life-altering commitments.
Also, don't marry a mentally unstable woman, especially if you are mentally unstable.
Mental illness runs in families despite their best intentions. These sick women are often the most exciting in your head and in your bed.
Do you like her parents? Do they seem like solid, honest people? Good breeding covers a multitude of sins, and remember your kids are going to get a lot of her DNA.
Don't try to fix people. I tried for most of my life and failed miserably. They remind me of my failure daily. If I had spent 1/4 of the time fixing myself as I did fixing other people, I would be more successful and happier. This doesn't mean you can avoid the often burdensome responsibilities of family and friendship, but establish just boundaries for yourself and firmly enforce them until it becomes second-nature.
Don't wait for the golden years. I have seen some of the best people struck down by diseases like early Alzheimer's and cancer. I've also seem some of the worst people living bitterly into their 90s.
You only have one life to live, so live full tilt and to the hilt. Visit graveyards if you get lazy, so as to remind yourself how short and precious this life is.
Trust me, you don't want to be almost 50 years old realizing you failed to take this advice.