To level your defense on RuneScape, you simple find a weapon, armour, and food and go train on monsters or *playersn that are the same level as you are. Just make sure you have enough food so you won't die and set the combat style to defense(which is Block).
If you are a high level like 40+, and have member status, i suggest you train at Soul Wars if you really want to train you defense fast.
For more information about the Defense Skill, click link to RuneScape wiki:
For more information about Soul Wars, click link to RuneScape wiki: