How do you know where to place the bar when a decimal is repeating decimal?

It is most correct to place the bar over the first set of repeating numbers. Some people teach a style that is also correct, showing one set of the repeated digits and then a second set with a bar over all the repeated digits.

This is not possible to show in this text-only format.

1/3 = 0.3 with a bar over the 3,

or 1/3 = 0.33 with a bar over the last 3.

1/7 = 0.142857 with a bar over all six digits, or

1/7 = 0.142857142857 with a bar over the last 6 digits.

Either format means the exact same thing. Your teacher may strongly prefer one or the other, or tell you to show your answer to at least 3 digits, or similar.