In an ideal transformer, there is no loss of energy. Hence,
Power supplied to the primary coil = Power used in the secondary coil
VpIp = VsIs
Vp = Primary voltage , Ip = Primary current
Vs = Secondary voltage, Is = Secondary current
(Remember, Power, P = IV)
Which brings us to the ratio of current which is
Is / Ip = Vp / Vs
This means that if the voltage is stepped-up, the current in the secondary coil is stepped-down by the same ratio.
Comparing the transformer equation.
Vs / Vp = Ns / Np
We ultimately get
Is / Ip = Np / Ns
(Ns = number of coils in secondary coil)
(Np = number of coils in primary coil)