"Just" has several meanings. Look up a good dictionary for details -such as "Longman
Dictionary of Contemporary English" or "Oxford Advanced Learners Dictionary of Current English". If your level does not match neither of these, you can look for a proper one from any of these publishers. Of course, you can make your own choice from what it is available in the market as well.

When "just" means "a moment ago", the present perfect tense is normally used in BrE.

A: Where's Eric?
B: He's just gone out.

In AmE, the past tense is common in this case.

A: Where's Eric?
B: He just went out.

I just had a phone call from Mary.

The expression "just now" is always used with a past tense both in BrE and AmE.

Sarah rang up just now.
I felt a sudden pain just now.

Hope this helps!