You're on!

[Theseus]

Used as a way of expressing agreement to something happening/gong to happen.
--"I'll give you fifty quid for your bike."
-- "You're on!"
-- I'll meet you here at 8 am.
--You're on!
This means that someone will be there.
--I bet you that you won't come out tonight to the party!
--You're on! :glare:
i.e they will take up the challenge.

 

[panadeli]

Μέσα! Έγινε! Πάει!

 

[Alexandra]

Two ways of expressing this "You're on!"

-Σου δίνω 50 λίρες για το ποδήλατό σου.
-Έγινε!

-Βάζω στοίχημα ότι...
-Πάει! (ή Πήγε!)

Edit: And of course "Μέσα!" as Panadeli said.