The simplest way to train combat(Attack, Strength, Defence, Magic, and Ranged) is to fight monsters. To fight a monster you simply click on them and watch your player fight them.
Attack- This determines how often you will hit a monster.
Strength- This determines how high you will hit a monster.
Defence- This determines how often a monster will hit you.
Ranged- This is when you have arrows, bolts, or knives(members) and use them to kill a monster.
Magic- This is when you use spells to kill a monster.(High Runecrafting adviesed)
My personal favorite places to train are Rock Crabs(Relleka) and Green Dragons(Wilderness).