Best way to build a multi site website?

I want to build a site where people can create their personal websites quickly. All their websites will be hosted on my single web application (i.e. there will be only one database which contains info of all the websites).

Each person will have an account where he can customize his website (template, content etc.). They can access their website from .mysite.com or mysite.com/xyz. I should be able to add different modules frequently, which will immediately reflect in all the websites created through my site.

Based on the above requirements, please suggest me if there are any frameworks to use which can ease my job. Also suggest me which language to use (php or java). Which one would be better.??

Thanks in advance!

Which one do you already know?

I am very much comfortable with JAVA