You would probably want to do this using subdomain and destination setup from your server you are using (Most of webhosting companies made this easy through their software e.g. cPanel). Directory structures is going to look like this. So when user was redirected t m.yourweb.com/ then it's not going to have m.yourweb.com/mobie.
domain [directory]
css [directory]
js [directory]
index.jsp [file]
purchase.jsp [file]
...
subdomain [directory] <- for mobile
css [directory]
js [directory]
index.jsp [file]
purchase.jsp [file]
So when mobile user was detected, JS is going to redirect this user to the subdomain (for mobile) instead of normal directory.
The other recommendation (which is better I think) is use the same logic from the above but when user was redirected to m.yourweb.com/ load css file for mobile version web instead of normal web. loading css through JS
If you think this is complicated, you might be interested in having a look at CSS3 media queries. This basically changes CSS layout depending on current users' screen. resolution Media queries tutorial