I'm not a dad (clearly!) - but I do have some advice, if you care to hear it.
It's been mentioned...hormones are a *****, and they can make your GF/wife be a ***** at times. Don't take it personally.
Change diapers. Assist with feedings - burping inbetween, feeding supplemental bottles at 2am, etc
Help with laundry, cooking, picking up the house, etc, at least for the first few weeks. 95% of her (mostly) awake times will be focused on that noisy, squishy little mess the two of you produced. Understand that it will need her attention more than you do during that time. Remember that you were there for the fun part - you need to be there for the parts that are less fun.
As your child matures and develops a personality remember that every child is unique, and the parenting that works for one child may not work for any siblings that come along.
Above all, boy or girl, spend quality time with your kids. Have tea parties, play catch, teach them to fish - DO stuff together. They will remember that far longer than the toys and electronics you might buy for them.
Be there...even if it's just sitting next to them while they process emotions.
Encourage them to explore, and let them fail. They will learn from it. Don't expect them to be perfect. You aren't. DON'T helicopter.
Love their mother, and love them.