We have all of these manuscripts and poems and epics recounting all of these amazing and complicated stories about their gods, demigods, titans, monsters and other creatures, such as in the Greek and Egyptian religions.
But how much of it was actually taken seriously by these ancient people?
I'm not sure how people back then were actually taught about their respective religions, so some explanation there would be useful. Like, did they have church or religious schooling, or was it just their normal curriculum? And in this context how much of this amazing mythology was actually taught and/or really believed?
Any info regarding the nature of religious belief in the ancient world would be appreciated since I really don't know much regarding it. :)
I'm not a specialist on religion, but in my research into Greek mythology and philosophy, I've picked up enough to get you started. I'm not able to answer your particular question about religious schooling, though.
Greek religion was based on orthopraxy (correct action) rather than on orthodoxy (correct belief). Essentially, what mattered was performing the right ritual in the right way, rather than belief.
There simply is no canonic text for Greek religion comparable to the Bible, Koran, Torah etc. When talking about myths, it's important to realise that there isn't a canonic version of the myth that is the 'real' version. Homer's accounts often differ in significant details from Hesiod's, and this is true throughout Greek and Roman literature.
So, did they actually believe? This is a very difficult question. Firstly, it wasn't particularly important to Greeks and Romans, although it seems of monumental importance to us after centuries of indoctrination in the importance of belief. There is a book on the subject, for the Greeks, at any rate, called Did the Greeks believe in their myths? by Paul Veyne (although I confess, I haven't read it myself).
The second reason it's such a difficult question is that we're dealing with a very long time period (from around the 7th century BCE to, for example, the 4th century CE when Emperor Constantine converted to Christianity) so it's extremely unlikely that attitudes to religion were unchanging for the whole millenium plus.