The first lesson -mounting and dismounting
Mounting and dismounting are not really a big lesson, but it's better to practice both safely. Getting on a horse can be done in the following ways: using a mounting block, stirrups, getting a leg up, vaulting and leg springing. All of these methods should effectively land you in the saddle.
The mounting block is the easiest to begin with (a mounting block is a little plastic step up block which gives you height to put your foot in the stirrup and swing your leg over the back of the horse - it is the easiest and safest mounting method). With a mounting block, you won't dig the horse in the side and you won't stretch your stirrup leathers. This is comfortable for both horse and rider.
Mounting using your stirrups, saddle and mane, can be challenging, especially if you are not the agile type. Getting a leg up is the second best way - this is where somebody boosts you up (same principal as the block) and you swing your leg over the horse. Nonetheless, you have better control over your horse. Vaulting and leg springing are for later when you're more experienced and you can display your talents.
Make sure you have control over the horse so he or she doesn't run off when you're half way on (it can be embarrassing, humiliating and painful).
Dismounting is always done with both feet removed from the stirrups and then swinging your right leg over the back of the horse so that you're almost laying over him and then sliding off gently to the ground.