You are trained in how to properly use a shield.
When you use a shield (except a tower shield), the shield's armor check penalty only applies to Strength- and Dexterity-based skills.
When you are using a shield with which you are not proficient, you take the shield's armor check penalty on attack rolls and on all skill checks that involve moving.
Barbarians, bards, clerics, druids, fighters, paladins, and rangers all automatically have Shield Proficiency as a bonus feat. They need not select it.