The easiest way to convert a decimal to a fraction is to let a computer do it for you. Most scientific calculators like those currently produced by Casio or Texas Instruments include a button or feature that allows instant conversion between decimals and fractions. Alternatively, free online resources such as calculatorsoup offer converters for easy public use.
Lacking immediate access to a computer, the following procedure is the easiest method for converting a decimal to a fraction. First take the fraction and divide it by one. For instance, if your decimal were 0.125 then you would write this as 0.125/1. Now, multiply both the numerator (the top number, 0.125) and the denominator (the bottom number, 1) by multiples of ten until all the decimal places in the numerator have been removed. In this instance, you multiply each number 10 x 10 x 10, resulting in the fraction 125/1000. Finally, you simplify this fraction by finding whole integers that both numbers divide evenly into until you have reduced each to a point where no further integers exist. In this case, both numbers are divisible by 125, resulting in 1/8, your final fraction.
Both general instructions as well as the specific work for any given decimal conversion are available at calculatorsoup's site.