Anything 12 volts or less is barely a drill in my opinion. You can use it to put small holes in soft material or drive small screws, but for the sort of things that are actually useful like carpentry, hanging sheetrock, or mounting electric boxes, they range from inadequate to pathetic.
18 volt drills are good for just about anything a corded drill is good for, but may be a little overkill for some tasks. Personally, it is my favorite size as it can easily do things that a 14.4 volt drill might bog down with, yet not ridiculously heavy or large at least not for me. Bigger ones are more than I have a use for.
Battery life and voltage are unrelated; battery life is determined by the size of the pack and measured in amp-hours. The 18 volt Makita I use at work has a smaller battery than some 7.2 volt drills. Also, battery type has a lot to do with it; that Makita uses a Lithium pack while the cheaper/older ones often use NiCad packs. I won’t go into too much detail about why NiCad is bad except to say that smaller packs with more endurance that are less delicate and require no special care are better than cheap packs that are heavier and require good charging/discharging discipline.
Extra batteries are a must. I always have at least one extra pack charged or charging.
As @RealEyesRealizeRealLies says, you also need to find one that is comfortable for you. Try a few out. I find some too small, others balanced wrong, and some just plain too heavy.
Lastly, don’t go cheap; you get what you pay for. A decent cordless drill may run a little more than you feel like spending, but cheaper ones will either not do the job or just break and require replacement. Buy right the first time!
@Adirondackwannabe Those are great for just making holes, but not so good as a driver, and I am not big on driving dozens/hundreds of long screws into 2×4s with a manual screwdriver.