Setup SPF and DKIM authentication settings

We are having issues with our email address being flagged as spam and I believe the issue lies within not having our SPF and DKIM set up correctly. I have a returned email that lays out the issue. See the attached txt file for copy of the error. We have our system send out invoices each month and a number of them get bounced back with a message similar to this. We would like to get this issues resolved.
We use Siteground as our host.