Open the preferences and change the ruler units with no files open.
You can also type dimensions in any field in any unit of your choice, as long as you also include the units, and ID will convert to the units currently in use.
thanks a lot Peter answer was really helpful, I already change it
but I have anew question about that dialogue box..
can i remove default paper sizes?
I was going to say yes, but in reading http://helpx.adobe.com/indesign/using/create-new-documents.html I see that since CS5 the New Doc Sizes.txt file no longer exists, and I haven't been able to find any sources that say there is an alternate method that allows removal of the default presets, so I think the answer really is no, but you can add additonal sizes.
An aternative might be to define the sizes you want to use frequently as document presets, which is really useful. These have page size, margin, column, bleed, etc, settings already defined so you just select the preset from the drop-down and you don't have to set anything else.
About this theme...
Can I change default page size?
In my version of InDesign cs6 default page size is 215,9mm x 279,4mm("letter") and I know, I will never use that size.. what can I do?
Go to File > Document Presets > Define... and select the [Default] style in the list. You should be able to edit it.