This can be pretty hard to do. If you have RCE, then you can definitely set this up.
In regular marketo, I'd suggest setting up a Smart List with the date of creation or date trial started with date range of Day or Month. Then use whatever other filters are necessary to monitor progress.
The challenge is getting that into a clear report, which won't be possible in regular Marketo. You'll end up with several monthly reports or smart lists, then have to plug that into SFDC.