If for example, I do something regular, and it counts as a prostration of worship in another religion, which I didn't know, does it count as a prostration of worship, and does it mean, I have to repent?
Prostration (sujod) is worship when your intention is to venerate a deity.
When Allah commanded everyone to prostrate to Adam =
And when We said to the angels, "Prostrate to Adam," and they prostrated, except for Iblees. He was of the jinn and departed from the command of his Lord...
This prostration in itself was not worship of Adam, it was simply something Allah ordered.
Similarly when Prophet Jacob (Yaqob) and his family prostrated to his son Prophet Joseph (Yusef) =
And he raised his parents to the throne and they fell down before him prostrate...
Neither of these two incidences were worship. They were simply acts of respect and not forbidden.
Therefore, when does prostration becomes worship? It's when your intention is to worship. And you don't worship unless you believe the object is a deity or divine.
So if you believed an idol was divine and you prostrate to it with the intention of worship, then it becomes worship.
If you're prostrating just for other reasons besides worshiping, then it's perfectly fine.