I just can't progress in the game since I don't know what to do

Every time I reach Attnam I have no idea of where to go. I look everywhere in the city and can't find who to talk to or where to go. I only found the prison, but it doesn't seem like I can do much there. I am seriously lost, and pretty tired of reaching Attnam and ending it there by attacking a random frog.

You have to deliver the encrypted scroll to Petrus. He's in the Cathedral, in the big throne room, at the end of the red carpet and a bit to the left. An old man with two servants around him. Talk to him and he will give you your next quest.

You have to talk to him twice, once to turn in the scroll, second to accept the next quest.

Make sure you don't lose the encrypted scroll. AFAIK it's indestructible, but if you drop it, you won't be able to proceed with Petrus's quest.