First find the LCD of the denominators then convert the fractions. Then you can subtract.
for example
2/7 - 1/4
LCD of 4 and 7 is 28 so we convert to fractions with denominator 28
1/4 = 1*7 / 4*7 = 7 /28
2/7 = 2*4 / 7*4 = 8/28
so 2/7 - 1/4 = 8 /28 / 7/28 = 1/28 answer