How do I find the fn+1 term for a implicit scheme?

I have a trapezium rule scheme that I need to apply to a differential function f(t)

whereby

yn+1 - yn = h/2(fn+1 + fn)

the initial condition is y(0) = 1

I therefore know yn and fn but how do I find yn+1 and fn+1

Do I use an Euler step?