I am actually doing this right now with a new gun I purchased that may become one of my EDCs- Beretta PX4 Storm D-type.
Step 1) Right out of the box, I took it to the range, and put it through the paces, looking at both accuracy, grouping, and reliability. I used 4 diferent brands of ammo (including a old box of Winchester Super X from Lord-knows-when). 200 rounds with no malfunctions with both magazines, and it's well on its way.
Step 2) Holster selection- as others have said, a worthy carry gun has to be able to be carried effectively. That means accessories, whether they be extra magazines, holster, etc. This is next on my list, and I'm looking at an OWB custom kydex for this one.
Step 3) Practice. Do I feel comfortable drawing, aiming, and using the gun. Does it feel "right" in my hand?