how do we know what modules we can undertake at level 1? is there a list somewhere?
For level 1 characters, City of Bywater is the best scenario to start with. Twin Sands of Time might also be possible with level 1 characters, but it will probably be more difficult (I think the recommended total character level is 15).
I recently played through City of Bywater -> Prelude to Pestilence -> Assault on Giant Mountain starting with level 1 characters and it went pretty well.
Basically every scenario expects you to have played City of Bywater. It was the only scenario that was originally included with Realmz.