i understand html, and css, but i don't understand the miva interface to apply the code. is it possible to create a fully customizable look to my store? if so how? thanks, =)

You should be looking in the Miva Store Admin under Pages, the templates for different areas of the store are located there and from there you can alter the appearance of the store to be customized for your needs. It's basically html, but you'll see some tags that have mvt in them, those are just the things that the Miva store calls.

Miva also has forums here
http://extranet.miva.com/forums/ if you want to check those out too.

i found the pages section. is there anyway to preview the changes that i make?

I don't believe you can preview by default, but it does store a history of the the versions, so if it doesn't work out, you can select the dropdown and go back.

I know there are modules/software that you can add into Miva so that you do have preview feature, but I haven't actually worked with any of those before. If you wanted to give it a try, there is Miva Mia http://www.mivamerchant.com/products/mia/ which allows you to run the Miva scripts locally.