You commit shirk if you believe that someone other than Allah has power. Or if you just say it but not believe it.
For e.g.: If a person has always believed in tauheed and knows that Allah has created him but commits an act which is shirk has he committed shirk?
Depends on what the act is, but generally if even the act is an act of shirk then yes, it can be classified as shirk, but a different type (there are different types of shirk, some bigger than others). Because i dont know what it is that you did, i cant really judge, but on this note i'd also like to say that Rasul Allah (SAW) said "Actions are according to intentions, and everyone will get what was intended. Whoever migrates with an intention for Allah and His messenger, the migration will be for the sake of Allah and his Messenger. And whoever migrates for worldly gain or to marry a woman, then his migration will be for the sake of whatever he migrated for." (related by both Muslim and Bukhari).
