Assume you have a following CSV file,
you can access to the specific column by specifying column number (data(:, colnum)). For example,
data = csvread('sample.csv', 1, 0);
f = fft(data(:,2));
csvread reads CSV file as matrix, but I would use readtable instead of csvread because it's much easier to access to specifc columns by using column names.
data = readtable('sample.csv');
f = fft(data.voltage);