Since your asking the question I will assume that this stuff is new to you and try to make it as simple as possible.
15 degrees is standard temp at sea level. If you are at sea level then the amount the temp is above or below standard is basic subtraction. If actual temp is 17 degrees then it is 2 degrees higher then standard.
If you are above sea level then the temperature goes down 2 degrees for every thousand feet of altitude. If you are at 5,000ft then standard temp at that altitude is 5 degrees. Take the actual temp and subtract it from the standard and you will have the amount temp is above/below standard.
All figures are celcius.
Please specify exactly what you are looking for, I think you might be confused. I imagine if you are doing this for flight planning your instructor wants you to find the actual temp at altitude for performance charts and cloud bases.
If you go to DUAT.com or a similar type site, you will find the actual temperature at different altitudes. At DUAT, I think they start temps at 6000. So, you'll have to make corrections to that altitude based on it getting colder higher and warmer lower. This isn't true with a temperature inversion of course, so look at other altitudes to be sure one doesn't exist.