How do I store values in variables

Read the FAQ #14:
How can I swap the contents of 2 tag fields?
Actions

Begin Action Group Test 2011#20110116.Swap.TITLE.ARTIST.1

Action #1
Actiontype 7: Import tag fields (guess values)
Source format: %TITLE%===%ARTIST%
Guessing pattern: %ARTIST%===%TITLE%
End Action Group Test 2011#20110116.Swap.TITLE.ARTIST.1 (1 Action)

Or use this Action Group.

Begin Action Group Test 2011#20110116.Swap.TITLE.ARTIST.2

Action #1
Actiontype 5: Format value
Field: TEMP
Formatstring: %TITLE%

Action #2
Actiontype 5: Format value
Field: TITLE
Formatstring: %ARTIST%

Action #3
Actiontype 5: Format value
Field: ARTIST
Formatstring: %TEMP%

Action #4
Actiontype 9: Remove fields
Fields to remove (semicolon separated): TEMP
End Action Group Test 2011#20110116.Swap.TITLE.ARTIST.2 (4 Actions)

DD.20110116.2200.CET