Hello there, I dont know if this is the right place.. But I new here...

I have a problem in my server in the domain part... Dont know why, but when ppl access a subdomain like for example: http://www.mysite.com/something it indeed open but with out the www. only http://mysite.com/something
I think that this is a lack of a configuration, just dont know what to do or where to go... Hope that some one can help me
any ideas?