High speed transformer

Hi everybody, I want to design a high speed transformer, what are the characteristic to consider, inductance, wire size ... I have difficulty to understand the mutual inductance, how do I calculate it ?
My transformer is for Switched mode power supply so it will will run higher than
30 kHz
I need to step down from
170V to 65V
and also need a
5 amp
output. can someone help me with this project?
thank a lot