Depending on the horse and what you are going to be doing with it there will be variations to the tack you will need. In general you will need a bridle, saddle and possibly boots. You will also need a numnah and girth. Some horses also require a breastplate or martingale.
Firstly put the reins over the horses head and leave them round the neck. This will enable you to have control over the horse while putting the bridle on. Stand facing the same direction as the horse and put your right arm under their head, taking the cheek pieces in your right hand. Take the bit in your left hand and using your thumb open the horse's mouth and push the bit in. At the same time lift the rest of the bridle to raise the position of the bit into the mouth. Put the head piece at the top of the bridle over the horse's ears. The noseband should be inside the cheek pieces and fastened. This should be fastened quite tightly as it prevents the horse from opening its mouth when being ridden. The throat lash should be fastened next and this should be loose enough to allow the horse freedom of movement of the head but is intended to hold the bridle in place if for example the rider falls and pulls the reins over the head.
The saddle should be fitted next; you might want to put the head collar back on the horse while you are doing this unless you have someone there to hold the horse. The numnah or saddle cloth should be placed on the horses back and the saddle should be placed on top. When fitting a saddle it is important that it does not restrict the movement of the shoulder and that it does not sit too far back and on the lumbar vertebrae. It is essential that you get an expert to fit the saddle and that the fit is checked regularly as horses change shape depending on the work they are doing. Pull the numnah or saddle cloth up at the front of the saddle to ensure it is not pressing down on the withers. Pull the girth through under the belly and fasten it to the girth straps. Pull the girth as tight as possible to prevent the saddle from slipping when you mount.
Consider if the horse needs boots, I generally ride my horses in knee boots if they are going out on the roads as these protect the knees if they trip over a kerb or slip.
It is essential that the horse has correctly fitting tack otherwise they can get problems, rubbed skin, tight muscles and back problems. If you are unsure then always ask an expert to check that the tack fits and that you have an appropriate bit and the necessary boots, martingales and added extras.