how can i choose my inductance?
Look at any switching regulator documentation... For that size, you do not need to make your own inductor, just order an already made one from Digikey or similar. Read through the TI design guide, it has all the math needed to size the inductor. It is not as easy as just 'picking' one. It is dependent on a lot of variables.

You want Ferite usually, Powder Iron is lossy at high frequency is is usually used for filters.

TI Buck Converter Designs

Digikey - Inductors

Also search this site more before asking next time:
Calculating size of cap &amp; inductor for buck convertor
LM2575 switching regulator - ideal inductor and COUT sizes