You use the Armed Combat Skill that you are currently using while fitting with a shield. Say, You use a sword and shield than you add Blades Skill + shield parry bonus to defend with the shield.
If you are looking to have a separate skill with a shield, you use Armed Combat:Clubs (Shield). This allows a PC to do a shield bash along with a sword strike, and it gives the PC a second option to parry with, but you lose the Parry Bonus extended by the weapon/shield.
Example (I have also included bonuses that are based on character attributes):
Let's look at a standard setup where a PC has a skill with a Longsword +9, but does not concern with having a shield skill. When he attacks with the sword, the dice modifier = +9 (skill rank) +2 (specialty) +1 (att mod) +2 (misc mod (from edges, or order abilities)) -2 (any penalties for flaws, wounds, weariness, physical conditions, or mutliple action penalties) = +12
If the same PC then goes to parry with his sword, he will have the following: +9 (skill rank) +2 (specialty) +1 Longsword Parry Bonus +3 Shield Parry Bonus +1 (att mod) +2 (misc mod (from edges, or order abilities)) -2 (any penalties for flaws, wounds, weariness, physical conditions, or mutliple action penalties) = +16
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Taking the same PC, but assuming he also has a Shield skill +9 for bashing. If he chooses to parry with the shield in any combat, he has the following: +9 (skill rank) +2 (specialty) +1 Longsword Parry Bonus +1 (att mod) +2 (misc mod (from edges, or order abilities)) -2 (any penalties for flaws, wounds, weariness, physical conditions, or mutliple action penalties) = +13
I have done a house rule that works pretty well. It makes some major changes to the basic combat system though. Each person has one attack and 1 defensive action each turn. (can be modified by traits). For the attack, you can launch a normal attack. If you launch an attack that requires 2 actions, you forfeit any defensive actions.
To defend against an incoming blow, you use 1/2 of your weapon or shield(for block), and then add the weapon or the shields defense to your roll. (plus any other applicable modifiers). In addition, if you have a shield, you can defend against two attacks, where as a weapon can only defend against one attack.
What this does is reduce the number of attacks that you can launch in a turn, and it also reduces the chance of a complete parry, but what ends up happening is that you end up reducing the number of extra successes. At the same time, a highly skilled person will be able to quickly overwhelm a lower skilled person, and also will be more likely to parry or block an incoming blow.
I have borrowed some of the ideas from other game systems such as GURPS and HarnMaster.