how to design the page irrespective of window size

Question

Hello,

I am creating a web application using asp.net3.5.
for that used css.when i run my application and resize the window  all the controls in the page are moving. and some fields are missing.i can't view my whole page data when i resize the window to minimum.
for some design part i used "%" and for some "px".is that the problem?
what shoul i do to get the whole page content when the window is resized to small as every webpage looks.

thanks,