How to configure domain for subdomain-hosting accounts

[Kibuyer]

Hello,

I have a reseller account which is handled by the Plesk Panel v. 9.3.
I purchased my own domain (let's say mydomain.com) and I would like to know how do I set it up in Plesk
so that I can create seperate hosting accounts with subdomains (in my purchased domain), for ex. seperate
client's account with address: john.mydomain.com. Of course the people who I would create
hosting accounts for would also be able to attach their own domains.

I added the domain in Plesk from my reseller panel, but it just got attached to my account, I can also
convert the whole domain to a complete client's account... Can't seem to find an appropriate option
for what I want to do.

Thanks in advance.