need to create custom sign in page

Hi guys
I am looking for a way for people to come to my site and upload some files from their computers
I need the upload page to look like my site

I am looking at this service: http://minus.com/
They are a file upload service

basically, I am trying to make it look like I am not using a third party uploading service so the user feels they are still on my site.

How can I do this? The site offers API (link at bottom of home page) but I dont know what API means, I just know maybe API is the solution.