How to configure DKIM for sending email from my domain

Hi

I have recently been spammed and as result the domain http://studiorel.co.uk has been effected and now all emails sent to gmail,yahoo,hotmail and aol users end up in users spam folder.

In an effort to fix the issue, i have setup private name servers, which point to the vps (and also because, cpanel required the server to be an authorative nameserver for domain). After setting up the name server, i am no longer getting any error messages in cpanel for email authentication.

I then also setup a DMARC dns record for the domain. However, it appears that when sending mail via smtp on the server, there are issues with the DKIM signature. I am getting reports from Yahoo, whom are sending a report with errors (i think for which i cannot make head or tail of).

So my quetsion is what am i doing wroing? Is there another step i need to do? why are the emails sent via smtp from the server not being signed with a valid signature?

I look forward to your replies.