I don't know the first thing about
gardening would probably be understood as the more emphatic of these two
statements if it's a written statement, and if it's spoken, both bolded words
(„first“ and „thing“) will be fully stressed, with a short but
detectable pause between the two words combined with emotionally coloured
appropriate intonation.
Both statements are stylistically „marked“ as emotionally coloured, of
course, because the „neutral“ expression of this statement would be
„I don't know anything about gardening“.