One struggle most of us have, especially when first starting, is how do we actually get on this giant animal to figure out how to ride it. Something that may seem obvious is that you need to be strong enough to lift you weight up over their back and center it with out falling over the other side. So in addition to being able to lift your own body weight you need to have the balance to stay up there. Those are just the basics.
As I witnessed just the other day, many beginners may not know that the generally excepted rule is that you mount from their left side. You would place you left foot into the stirrup just far enough so that the ball of you foot rest just on the center of the stirrup. Be sure not to jam your toe into the horses side giving it the que to move. The next trick that many people do necessarily realize is that they should always be aware of the reins in the event your horse moves. Next you would also want to naturally grab the back of your saddle in an effort to pull yourself up. Well guess what, that just shifted your saddle making it sit uncomfortably on your horses back. Instead you should reach your right arm across the back your horse to the other side of the saddle and use it to balance your weight as you mount. As you see in the above photo you also need to be aware of your right leg as you swing it over your horse. If you have trouble in this area, it is usually that you karate kick your horse in the butt or you end up dragging your leg across it. Both are extremely dangerous if your horse is not used to this kind of behavior (If you can believe it some actually do get use to it).
Please keep in mind that it is never safe to get on a horse in a saddle that is not cinched on. DO NOT TRY IT. It ends up very badly for the average person.
Another thing to note, is that if you are going to use a mounting block make sure it is stable and close enough to the horse and NEVER dismount onto it. Remember SAFETY FIRST. Horses can be unpredictable even when they are well trained and tolerant of our imperfections.