The carrying part I really can't answer, besides this: get a good belt, and find a holster you like. Everybody --and every body
--is different. We may be the same height and weight, but...unless I'm your twin (and perhaps even despite being your twin) , I may have a holster that is the most comfortable thing in the world, yet you can't abide.
This is why people who have been carrying for a while end up with a box of holsters. Mine is a 13gal Rubbermaid container full of 'em...
As for ammo, though...target wise, buy whatever's cheapest. Odds are, your XDm will eat it just fine, and the practice you get pulling the tirgger more often will make you a better shot than trying to find the perfect bullet.
But when it comes to defensive ammo--I suggest you read the following:
Best Choices for Self Defense Ammo
Thoughts on Service Pistols, along with Duty and Self-Defense Ammo Recommendations - M4Carbine.net Forums
Choose any one of the rounds on either of those lists--bearing in mind that some of the rounds listed may be considerably more expensive and/or harder to find than others, but ehy all will perfrom pretty much the same. Buy a couple of boxes of your chose round, and take it to the range. Shoot at bare minimum
one entire magazine full out of every magazine you plan on using for defense. If it runs 100% reliably, and is both accurate and controllable in rapid fire, buy a few more boxes, and be confident that you have a proven load that will work for you in your pistol...and don;t worry about what other people use...
Because just like holsters, one size does not fit all when it comes to carry ammo.