Start on the right side, in the ones group, and subtract the bottom number from the top number. If the top number is smaller than the bottom number, you're required to "borrow" 10 from the 10s group. For example, subtracting 125 from 133 would look like this:
133
125
You can't subtract 5 from 3, so you have to borrow 10 from the second column. Cross out the top middle 3 and replace it with a 2. You will then have 13 sitting in the ones column and be able to subtract the 5, leaving you with 8.
Move to the 10s column and subtract the bottom number from the top number, keeping in mind any changes made to the top number while subtracting the ones group. If the bottom number is larger than the top number, you would have to borrow again -- this time from the hundreds column.
Subtract the bottom number from the top number in the hundreds column. This will be your final step, and you will get your answer.