There are a few basic skills you will need to learn when first learning to ride. These are building blocks of becoming a good rider.
The very first thing you will want to understand as you learn to ride is how to halt, whoa or stop. Your instructor may use any of those words when they want you to bring your horse to a standstill. Most likely, your first steps as you learn to ride will be the walk. But before you do that, you need to know how to stop.
To cue for a halt, close your fingers and squeeze backward. The horse should stop as he feels the backward pull on the reins. As you use the rein aids, you will eventually learn to stop by using your body, seat, and legs. By stopping your body, you are cuing your horse to stop as well.
As you gain skill and refine your aids you will push your seat deeper into the saddle, stiffen your back, close your legs on the horse slightly, and squeeze back on the reins. As soon as the horse responds and comes to a complete halt, the cue should be stopped. This doesn’t have to be forced. It sometimes helps to exhale as you ride into your halt.
Sometimes you will need to apply a stronger aid, pulling backward if the horse is reluctant to stop. "Give and take" as the horse takes strides, squeezing back and easing up until the horse halts. You can also ask your horse with your voice to "whoa". Keep in mind, if you’re heading for the show ring, you won’t be able to use voice cues when you are riding in a horse show. Your cues for the halt should be smooth. There should be no 'dead pull' or jerking and jabbing on the reins. Ideally, you want your aids to be almost invisible.
If the horse has halted correctly he will have his nose down, will not swing to one side, and will be standing more or less square (a leg 'in each corner'). If the horse flips its head up, you may have applied the cue too sharply. If the horse swings or turns, you may not be holding the reins evenly.
Tip: Don't forget to breathe! As you are concentrating hard you may find you are holding your breath. Breathing naturally will help you stay relaxed in the saddle.
Once you've come to a complete halt or stop, you may be asked to walk on, trot or jog or even canter or lope depending on how advanced your instruction is. If you have come to the halt because your ride is over, give your horse some praise, perhaps a scratch or pat on the neck. The dismount and loosen the girth or cinch, and if your ride is over run up the stirrups.
Like any new skill, learning to halt will take time until it feels comfortable and natural. Eventually, it will come automatically and your aids will be more effective and unobtrusive.