The same way you level up your other combat skills, either the Soul Wars Mini-Game or Fire Giants at Baxtorian Falls.
Raise your defense man, RAISE YOUR DEFENSE
I'd sujest hill giants
Fighting more, maybe to to the graveyard.
Make runes. Air, Water,...
40 Defense? sure why not? =)
Castle Wars Armour:Red - Has Defense equal to Steel.Silver - Has Defense equal to MithrilGold - Has Defense equal to Adamant
99
raise defense
Try to do stuff that requires your current level.
In RuneScape? No, smithing does not affect your combat level. Your combat level can be affected by the following skills: Attack, Strength, Defense, Constitution (formerly called Health), Magic, Ranged, Prayer, Summoning (for members). Search for an online RuneScape combat calculator to quickly see how the different skills affect your combat level.
The best armor to wear in Runescape I would say is Bandos, mainly because a nice combination on Strength bonuses with Defense.
Prayer, Magic, Attack, Defense, Strength, Range, and Constitution all affect your Combat level (the level that people see when they roll their mouse over you). You can raise any of these to help you, as you put it, level up.