I'm trying to start a game in the year 500 AD as Prussia in Europe and have absolutely no clue how to expand my country other than going to war with my neighbors. I cant seem to move troops into land that isn't discovered and cant seem to take over an empty provinces due to not having a colonist or being able to choose a native policy.

I am not crossing the sea to SA or NA to colonize...how in the world can I take over the empty province next to my country then? Please help, thank you!