Ciao Daniele- 

I think Mateus covered things pretty well, I just wanted to mention that "just" has an entirely different meaning as well, related to justice.  To be 'just' means to be right or fair, and 'unjust' is the opposite. This meaning is used less commonly.  One example sentence:

Some believe that American society is just, since all citizens have individual rights.

Ciao Daniele!

Here's some examples: 

Use 1 – Just meaning “simply” . It’s usually used at the beginning of the sentence.
A: I don’t know if I should apply for the job. There’s so much competition.
B: Just go for it.
Use 2 – Just meaning “only” . It’s usually used at the beginning of the sentence.
A: Who’s coming to the party?
B: Just me.
Use 3 – Just meaning “exactly” . It’s usually used after the subject.
A: Am I late?
B: No, you’re just in time.
Use 4 – Just to mitigate the force of a question or request . It’s usually used before the main verb.
Let me just ask you something…
Use 5 – Just to politely announce intentions. It’s usually used after subject pronouns and auxiliary verb.
I’m just going to use your bathroom.

Hey, no problem!  When we say something like "I just got up" it could have a few meanings depending on the context.  Here are there two meanings I can think of:

1. I got up a little while ago.  example: "I just got up, so I'm still pretty sleepy".  Or: "Since you just took an english class, you must speak very well!".  You can also say this sort of thing to clarify that your action was in the recent past: "I only just got up" or "I just recently did some traveling in europe".

2. I got up because I didn't want to lay down any more.  example: "I was laying on the ground looking for shooting stars, but I didn't see any, so I just got up".  Or another example: "I didn't see what I wanted at the grocery store, so I just left".

I'm only realizing now what a tricky word this is!  I hope these examples help.