How to I connect to your own dropbox account in my apps??

My questions title is quite difficult to understand....

My question is

I want to develop application to use dropbox api.

In my apps, users can upload/download files to their own dropbox account.

if it is possible, how can i get the users appkey or permission to connect their own dropbox account?

I mean that my app is linked-ring of dropbox.

so, when i test my own account, i create app console and assigned my appkey, and write my appkey in the code...

in my apps... my users don't want to such step... so how can i linked users drobbox account in my apps?????????????

I'm sorry because my english is wrong..

anyway please comment to my questions...