Not all calculators are the same, so I am going to provide a few examples of how to calculate the arctangent of a number (say, 6).  The arctangent of 6 is 1.4059... (make sure you have selected radians, not degrees).    So that is the number you will get if you perform the operations correctly.

If there is a button on your calculator labeled "INV," this usually means "inverse," which functions in the same manner here as "arc."  So we would try

6 INV TAN

Sometimes the ARC button is used instead of the INV button.  In this case,

6 ARC TAN

If you see that the TAN button has a little "tan^-1" tag on it, then that means you must use some sort of secondary function button to get to the arctangent function.  This secondary function button is usually labeled "2nd."    So

6 2nd TAN

Those calculators with the large screens that allow you to type in mathematical expressions directly will use a different syntax, something like,

2nd TAN ( 6 ) ENTER

or

2nd TAN ( 6 ) =

Finally, if none of the above work, consult your manual, or ask a knowledgeable friend!