If you are certain you only need one bus from Euston Station...
If you already have an oyster card, you'll need to top up with £3 (£1.50 there and back).
If you don't already have an oyster card, you'll need to buy one for a £5.00 deposit (which you can get back if you return the oyster card at a ticket machine + any excess credit on it) and top up with £3.00.