To create a report on Average First Reply Time, first navigate to Reporting > Insights > GoodData > Reports and click the "Create report" button:
Then add a new first reply metric by clicking "Add New Metric", matching the following settings, and clicking "Add":
This will be your 'What', the thing you are measuring. To make the report more meaningful, you can either filter for a specific date range or slice the data up to see trends. The 'How' is where you will slice the data. If you want to see the Average First Reply Time by day, you will use Date (Ticket Created).
This will give you the Average First Reply for all tickets created per day for every day that a ticket was created. To make this more digestible, you can filter for tickets created this month. To do this, select the first Filter option "Select from a List of Values (including date ranges)", and then match the following settings and click Apply:
This will give you a table, but this data will probable be easier to read in a line chart. To do this, select the symbol at the top right of the report editor and voilà, you have an interactive line chart showing you the first reply time over a series of days: