Work on cardio fitness - running, extended hiking on hills, and cycling are good.
Some 14ers arejust long walks' others are highly technical by their easiest routes. on your pproach, spend some time at a reasonably high elevation before your ascent. "Climb high, sleep low" is a good mantra.
Get some first aid training and learn to recognize the signs and symptoms of altitude sickness.
Above all, have a good time.
Did my first 14er at age 51 last summer - East ridge (class 1) of Quandary Peak in July. Would recommend it to others as a first as well. As implied, this was definitely a "long walk" vs technical, but still not a cakewalk. Def spend a few days at higher altitudes before starting. We stayed in Breckenridge (9k') which helped acclimate. And cardio training prior is a must, esp if you live at much lower elevations like me.
Everything @hikermor said is good, and I'd add that you need to be educated on the mountain overall, your planned route and the season you'll be trekking in. Maybe start-off on a few class-1's (Quandary, Greys/Torreys) before launching on something more challenging/technical. I found the website 14ers.com to be a great resource with route info and reasonably current condition updates from other climbers.