Yeah, um, you have to change the fractions. Basically, on your example, multiply the 2 on the bottom by 6 to get 12 on the bottom, and therefore you have to multiply the top by 6, so you get 6/12. Then you simply do 7/12 minus 6/12 to get 1/12. Hope this was helpful!
In response to the above answer this brings back memories from school. Is it something to do with finding the lowest common denominator or something? What I'd like to know is despite struggling with these things at school I have never, since leaving school ever had the need to take 1/2 away from 7/12 so what's the point of it all? I just think teachers enjoy causing kids great stress and worry!!
There is an easier way 6/12 =1/2 so you are left with 1/12
maths is not taught in such a rigid way these days - if an easier way of working out is available use it!