Soldier Shield: How do I get one?
This is relatively simple. If you meet the HP requirement (75HP) simply walk up to your faction leader and say:

You say, "I want to become a Soldier".

Your faction leader will then order you to kill faction soldiers (the NPCs that defend the flagpoles) belonging to opposing factions.

Kill the first soldier, and then return to your leige. Say anything to alert them to your presence and they will give you information on the other soldier you will have to kill. Do this, return, and they will finally make you a soldier.

Taken from "Quests" FAQ here:
http://gilcon.net/meridian/faqs-get.asp?ID=98