This is a subject on which I am not an expert, but I will share with you the tips I got from other folks (and la internet) on getting Zapa to take to the bit.
First, make sure your horse knows how to lower his/her head. You should always bit your horse from this position. A low head = relaxed horse.
Before putting the bit in his mouth, remove the reins. They can get in the way, especially for a baby. If you have to remove them when he is on, it is adding pressure to something that is already pretty new.
Give him a treat, then show him you have a second one and put the bit in his mouth, give him the treat.
Note: I ride two of my horses in bit hangers (just a piece that goes behind the ears that is connected to the bit), but for Zapa I put it on a bridle. I had it fairly loose, but not so loose that it would bang his teeth. I did need to do a little adjustment. Also, NO cavesson. It needed to be as simple as possible to start.
After the bit is in his mouth, let him graze (or eat his dinner grain). I started with 5 minutes and now he can be in there for 30 minutes or so eating dinner. I started with a loose ring 3 piece snaffle, although I am going to start him in a level 1-2 Myler bit (as he is a super sensitive horse, he doesn't need quite so much as a snaffle I think).
After a few weeks of that, I started to lunge Zapa while he was carrying the bit, with the leadrope attached to his halter. The next step was side reins to help him learn to give to the pressure of the bit (which took him about 3 seconds to learn) without my hands getting in the way.
Next step is to drive him with the bit and lunge him with long lines (two lunge lines, one on each side), but we are still some time away from that. After that I will start to flex him each way with both the bosol (like a halter with reins) and bit rein together, but bit rein loose. I'll ride him that way for quite a while before just relying on the bit rein.
Some of these ways are specific to a Paso Fino but can work with all horses. If you are using a rope halter, you can ride with that and have the horse carry the bit for a while.
But remember, make it comfortable. Your relationship with your horses mouth will be a vital part of your relationship. So take the time to do it right!