https://doc.sitecore.com/users/81/sitecore-experience-platform/en/the-conversions-tab-reports.html#UUID-830dcc82-f326-b8af-3d36-38dc92c63719_id__Goals

In the Experience Analytics, in the Conversions tab, you can get the goals reports. Not sure what is exactly possible in 8.1 (the link above is for the 8.1 specific docs, so the report is there), but in a 9 version I can easily filter on the goal and the timeframe and see all analytics.

If this ootb information is not sufficient, you'll need to create a custom report but I would start with checking the ootb information.

https://doc.sitecore.com/users/81/sitecore-experience-platform/en/the-conversions-tab-reports.html#UUID-830dcc82-f326-b8af-3d36-38dc92c63719_id__Goals

In the Experience Analytics, in the Conversions tab, you can get the goals reports. Not sure what is exactly possible in 8.1 (the link above is for the 8.1 specific docs, so the report is there), but in a 9 version I can easily filter on the goal and the timeframe and see all analytics.

If this ootb information is not sufficient, you'll need to create a custom report but I would start with checking the ootb information.

https://doc.sitecore.com/users/81/sitecore-experience-platform/en/create-a-custom-experience-analytics-report.html