I agree with ze_crypt, however dont become p2p until after already having at least a decent amount of cash (maybe like 3 mil+) that way you can be a defense pure in f2p and a summoning tank in p2p
Also rune chainbody is the best item to wear... and once you get 60 def you can wear dragon plate
Use an iron dagger and the best possible armor.
If you get 75 defense and are p2p, use a dragonfire shield ONLY WHEN FIGHTING IN PVP. This gives magic exp versus monsters, but none versus players (so I've read)