.httacces and virtual subdomains

[eluser]jurosik[/eluser]
hi folks, i need to use virtual subdomains and use it as a variable, like:
http://var.domain.tld would use /default controller
http://var.domain.tld/foo would use /foo controller
and so on..

i'm totaly lost in these .htaccess expressions

please, is here anyone who could help? thx