Well instead of just giving you the answer, how about I explain it. You must first get a common denominator which you can get by multiplying the two together. 1/3 = 8/24 and 1/8 = 3/24. Now that you have a common denominator, you can simply subtract. 8/24 - 3/24 = 5/24.
The answer is 5/24. First, you need to find the common divisor of the denominators which is 24. Then, divide both 3 and 8 and multiply by the numerator which is one to get 8/24-3/24. When both fractions are subtracted you get 5/24.