At times, you may substitute "only" for "just", but they do not have to be equal.

She is just a naive pupil = She is only a naive pupil.

Just has a few of meanings I mention below:

it means exact, precise
That is just what I want to know. 

It means only, merely
He is just a child.

It means hardly
The skirt comes just below my knees.

It means about to
He's just about to leave

it means far and away
The play was just splendid.