From the wording of your post, I'm guessing you're not yet very familiar with how a switched mode power supply works.
I'd respectfully suggest it will be really, really worthwhile for you to do some reading up on how a 'buck converter' works in a lot more detail, before you try and design your own converter. You'll save yourself a lot of frustration in the long run, I promise.
The best learning tool I know of for the purpose is LTSpice, from Linear Technology. They make a wide range of converter ICs, and there are simulation models in LTSpice for most if not all of them. Use it to learn about inductor current, power dissipation, parasitic effects, loop stability, ripple current and bulk capacitance.
Designing a good switched mode power supply isn't easy; you might well find it's better to just buy a pre-assembled module off the shelf.