How to Total all Records in a Field?
The quick and dirty way to show totals is to add a Totals row to any Datasheet View of a table or query.  First open the Datasheet, then on the Home ribbon (i.e. toolbar tab) click the Totals button (with the sigma sign).  A Total row with empty cells appears at the bottom of the data rows .  Clicking on a cell reveals a drop-down list with options for various aggregate functions, include sum.  See this documentation page.

A more advanced way to get totals is to create an aggregate query.  Within the standard Query Design View, click the Totals button on the query Design ribbon.  This enables and shows a Total row in the design grid.  For each column of the query, Totals options can be selected including "Group By" and various aggregate functions.  The same query can also be edited in the SQL view which will show a GROUP BY clause and various aggregate functions in the column SELECT list.  See this documentation page for further details.

Asking questions here is fine, but especially for beginning topics as this, the web contains many tutorial pages with much more detail, just as I linked to.  All I did was search for "Access sum fields" and "Access query total" to find those pages.  I suggest doing those same searches so that you can see various other resources and tutorials.