[TYPO3] set up TYPO3 works with subdomains

[Rocky Ou]

Hey list,

I have 2 websites done in a TYPO3 installation. I would like to set the 
domain name for first website as www.mydomain.com and the second website 
as www.sub1.mydomain.com.

Can any of you help me out on how to set this up please?

Thanks a lot in advance!
Blessings,
Rocky