The casters have more ad and less health than the melee minions. If you want to push the wave you kill the casters as fast as you can before touching the melee minions and if you want to freeze your lane you only last hit. Depending on your champ, you wait until the last possible moment to hit each minion. A good way to practice this is to go into a practice game with no runes or masteries, no items, and no abilities and just sit there focusing on only last hitting to freeze your wave.
Another little trick is that if you want to roam or leave your lane for teamfights/objectives is to kill off a single caster minion from a wave. This will cause the waves to meet in that spot for a while and your minions will gradually grow into a super wave of 30+ minions. This will force the enemy to either leave to stop the wave leaving your team with a 4v5 or they will lose a tower.