“Just tell me the truth.”

You find out that the disk tray of your DVD player is broken. You suspect that your 6-year-old son broke it. You ask him if he broke it, and he doesn't answer but looks guilty. You want to encourage him to confess, so you say:

Just tell me the truth.

just (do something)

This is an order to do something. You're telling someone to do only what you ask, without discussing it or protesting. Some examples:

Just shut up.

Just listen!

Can you just stop it please?

tell (someone) the truth

This is the phrase that people almost always use in conversation to mean "saying what really happened" or "not lying".

You use this phrase when you think someone is lying, or when they're trying to decide whether to lie or tell the truth. Examples include:

I wish you'd just told me the truth in the first place instead of trying to cover it up.