If I understand you question correctly you are asking for:
1/4 - 1/12 = ?
If so do this:
First find a common denominator for both fractions:
since 4 goes into 12 three times, multiply the numerator and denominator by 3:
3/12 - 1/12 = ?
Now you can subtract them from eachother(subtract numerator & keep denom):
3/12 - 1/12 = 2/12
Finally reduce your answer to the simplest form:
2/12 = 1/6
1/6 is your answer.