One day your horse is going to need to wear a bit. If you are raising a foal, in the first few days it is useful to start putting your fingers into its mouth across its bars. When it is weaned a little string can be put across the bars to get it used to the idea of having something there. I don't recommend anything stronger at these ages, because the bars are really quite sensitive.
Once the horse is a late yearling, or an early two year old, you can tie the string to the halter and lead it around pulling lightly on the bars. Just to get the horse used to the idea. String doesn't taste the best, but it is not repulsive. If it is chewed it wont harm the horse, either.
Then about two and a half, a sweet iron bit can be introduced. If the weather is cool, it is good to hold the bit in your hand for a few minutes to warm it. If it is too cold there will be resistence. We have been trying to avoid resistence, and since this part of the horse's education is so important, it is worthwhile just going really slowly.
Just slip it in, but be careful not to bang the teeth. If the horse wants to just spit it out, let him, at first. This gets him used to unbitting without hitting his teeth. Next time you put the bit in, leave it there for a minute or two before allowing him to spit it out. Make sure you praise and scratch him for being good. This is really important as you want the horse to know it has done well. Most young horses want to please you, and if they know they have done well, will be willing to repeat the positive actions. After a few in and out, it will be time to put a bridle on. If you have taken time to halter train the horse, getting the bridle on should be no problem.
The bridle should not have reins attached to it at this stage, just the cheek straps, the troat latch and the poll straps. You could put a forehead band if you really wanted to, but as you are just getting the horse used to the idea at present, it isn't necessary.
Make sure the bit is sitting where it should be in front of the first back teeth and on the bars.
Leave the bridle on for about 10 minutes or even if you wanted to, for a feed; however, just grazing with the bit on at first is better. The undo the straps and let him spit it out again.
After two or three days, repeat this until he accepts the bit in this way without fussing or trying to scrape it off/out. Some horses accept it faster than others. Once you are both happy with the bit, tie a piece of string on each side of the rings of the bit. Pull softly one one string till he gives his head in that direction and then the other. Keep doing this softly over a few days until he gives his head almost to his shoulder on each side. It just gets him used to the idea of the bit being a guide for direction. You can also pull down on both strings together so that he puts his head down and in towards his chest. If you had taught him the word stop, now is a good time to use the word as you pull softly.
After each session make sure you tell the horse how good it is and scratch it in its favourite place. Some sort of non food reward food is appropriate, because the horse is learning to give to your commands.
Once you are happy the horse is responding kindly to your pulling on the bit, you can replace the string with reins. Then you are ready to do leading and other ground breaking with your horse. Take it easy and slowly, building his confidence and not letting resistence happen. In no time you will find your horse ready to respond to aids from the saddle.