You don't need to be prot to do instances.
In fact it makes it harder to do instances at your level as prot; prot spec requires some obscure mechanics to hold threat. But if all you want to do is press sunder and revenge sometimes... But you can do that as any spec.
Arms is probably the stronger leveling spec, and is easier to hold threat non-heroic non-raid.
Spec? I would concentrate on getting to 70, before considering specing prot.
SAVE ALL YOUR DEFENSE/PARRY/DODGE/BLOCK/BLOCKVALUE/ETC... GEAR.
SAVE ALL YOUR FIRE/ICE/ARCANE/SHADOW/NATURE RESIST GEAR.
SAVE ALL YOUR DPS GEAR.
You can always respec to anything later. But the gear makes a huge difference...