Welcome, Mary,
Lightly scuffing the paint with some 100 grit sand paper would be your best bet, but no need to sand through the paint. Just scratch it up a bit. Shouldn't need to spend more than 30 minutes on it.
A good idea, too, is to check to see if your counter top(s) is level first along the back edge, which may dictate how you set the bottom row of tile. In your planning be sure to remember to leave a gap of 1/8" +/- between the counter and bottom row.
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Dan
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If I recall correctly my memory is excellent, but my ability to access it is intermittent.
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