Do research on "responsive design". If this is a home-grown application, or a heavily modified canned application, it will be up to you. If it's an off-the-shelf canned application, you may be lucky and find a plug-in/modification/add-on/theme that does the job. If not, do it yourself.
(This was apparently originally intended as a spam post [thus, @Pete's edit], but it's still a good general question that it would be useful to answer. I will change the subject line in the original post.)