Your horse is tacked, double check the girth and prepare to get on. When you raise your foot to the stirrup use care to keep it pointed towards his head, rather than into his side. If your foot should move and you jab him then he'll move, then get corrected for moving. With your toe facing his head, some movement doesn't poke him. Gather your reins and grab a handful of mane or the horn for balance - straighten your leg and in one motion swing your leg over and gently sit down. Don't "flop" into the saddle or on the horse's back like a thud of a sack of potatoes. Instead sit down as if you're in a chair you're not sure will hold you - easily as if testing that it'll hold. With your right foot secure the other stirrup.
Give a gentle side to side shake - as if settling into a deep cushioned comfortable seat. Get your feet in place, get those heels down and check your position in the saddle. Gather your reins properly, with light contact with the horse's mouth. You don't want a snug hold or a death grip, nor enough play in the reins you must grab desperately to shorten them. Light contact means you move your hands a few inches and it's connecting. You don't lean and get out of balance.
You're on, you're settled, you're collected - time for forward motion! Squeeze with the lower leg firmly and say "walk." Some horses a slight tug on one rein will encourage them. As soon as he begins to walk stop giving cues and let him - if he stops repeat the cue. You want control so it's important to take it, even at a walk. Don't let the horse just wander aimlessly. Pick a point and go! Lay slight lower leg pressure and turn your horse around the leg. Use large circles, serpentines and figure 8 patterns. Make your cues clear and with authority. This doesn't mean being abusive. It does mean following through. If you threaten with a whip be prepared to give a tap with it, or the horse knows you're simply bluffing. This gives him control and, more importantly, plants the seed to leave you out of control.
As you're comfortable, your horse is calm and quiet and following your direction, bump with the lower leg and say "trot". When he does let him do it. If he gets charging ahead to a canter don't snatch on the reins - use *one* rein and pull him around and repeat "trot!" Remember you can ride in large circles as long as he can lope them! Keep circling - when he's calm and relaxed and at the speed you desire you can straighten him out.
If you're riding English you'll need to learn to post...remember the adage "rise and fall with the leg on the wall." When that leg is in the air you should be up, then sit as the leg comes down. This takes practice! If you're having a hard time seeing the legs take two different colored polo wraps - for example a white one on the left leg and red on the right. You can look down without leaning and easily see the leg because of the wrap. This also works for leads.
When you begin to canter there's somewhat a discipline/breed difference. While generally you'll apply leg pressure with the outside leg and slightly more forward pressure with the inside, remember to shift your legs and let the pressure off when he canters. Some teach to turn the nose slightly to the wall of the arena, more common in Saddlebreds and some other show breeds. Performance people often tip the nose to the inside as in circle patterns that is the lead they want the horse on. Whichever way you train be consistent.
Riding is fun and good exercise. Riding well means it's comfortable for you and your horse.
Having your horse tacked and ready to go is only part of preparing to ride. The bit and reins communicate but so does your legs, weight and mind.