It depends on the horse...
Based on my experiences when I started my gelding...
For example what should the horse know how to do? Whether undersaddle or not, in my opinion, a horse should know how to lunge at a walk and trot and know the voice command/cluck/click/kiss noises for walk, trot, and whoa. Basic ground-driving, though IMO, that isn't a major thing and can be taught anytime, have good ground manners, be respectful...
What do you make sure it knows? Lunging and acceptance of pressure on the sides, basic voice commands, groundwork...
What age do most people like to start? I started my boy at two years of age, though it all depends on the horse...
Should the horse be ridden for the first time without a saddle? Depends on the rider and the horse...
Should it have a bit on? Or is a halter and lead ok? What do you use? I got my boy used to a bit months before I ever got on him, but his first handful of rides were with a halter and lead, I only introduced the bit after he was doing well in halter and lead... I still switch him back ot halter and lead because, since he was started in it, he responds to it just as well as he does with a bit...
Does anyone have a special technique they use? Absolutely no treats whatsoever... if you give a reward, let it be in the form of a scratch or 'good boy' or pat (in my experience, treats make my boy mouthy and I hate mouthy horses), lots of in hand walking (I mean miles and miles of in hand walking... I've found that it really helps to form bonds)... also, on a very nervous horse, it might be a little better to have a 'steady eddy' around, as the calm disposition of an older horse who you and the youg horse trusts may help calm the younger horse...
Anything else before the first ride.....? Desensitizing to saddle, stirrups flapping, loud noises, anything!!!!