Ok guys I found the way to the Knight's place and access to attack the slimes. It's quite easy but because the game doens't say anywhere is just a mess:
1. You need to unlock the Wolf's tavern serving job. There you will serve Morris and talk. The first talk menu will be the 7 contribution one. YOU NEED TO TALK TWICE.
2. Go to a borad message thingy, cancel the slime mission and you will have access to a new mission. The Bornide one.
3. Bornide (or whatever is spelled) is like a seller that sells pictures of girls. The mission tells you to go to the shop (is located in the starting plaza, you will see a $ green mark floating). Go and buy the new girl's picture, the first cheap one is enough.
4. Turn in the mission, accept the slime one again. Now go to Wolf tavern and to a shift again. You will talk to Morris once more but now you have 8 contribution points to talk about the second topic.
5. After that, a new mission at the guild's tavern will be available (in the starting plaza). Go there and it will trigger a delivery mission to the Knight's palce.
6. Knight's place it just around the corner of the starting plaza, is a big wood door (there will be a yellow marker there now). Talk to Shienna and she will tell you to get gut and kill some slimes.
7. Now you can attack slimes of level 1. After you kill 3-4 slimes of level 1, you can attack slimes level 2, and after some more kills level 3.