Dizzy about dependent dropdown

I´ve watched the video tutorial again and again, but I didn´t get the dependent dropdown box working. For my accounting-software I need two dropdownboxes: One where you can choose, if it is an income or an expenditure. The second dropdown should depend on the value of the first and display different accounts for the both kinds.
I´ve tried for over six hours now, but didn´t get any result. By now I´m so doubtful I even don´t know how many tables I need to do this. And do I have to link them at the "datasource table" display? Do all tables need a common field to link?
Sorry, that must sound like I´m absolutely crazy...and somehow I think that might be right. My head feels like spinning around and I don´t find an answer. So please, if anyone answers, do it like to a baby <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=11566&image=1&table=forumtopics' class='bbc_emoticon' alt=':rolleyes:' />