If you have problems dealing with negative numbers, you can use various "tricks". If you think of walking forward as positive or plus and walking backward as negative or minus, 7 could be seen as 7 steps forward. The first minus tells you to turn round and walk the other way, or to walk backwards. The second minus tells you to walk the opposite way, so you turn round again, and you end up walking 12 more steps forward. If you were going to walk backwards after the 7 steps forward, because you saw the first minus sign, the second minus sign tells you to do the opposite and walk forwards instead, so you end up having taken 19 steps forward altogether.
Another way is to think of a thermometer where zero means freezing and positive numbers are degrees above zero and negative numbers are degrees below zero. 7-(-12) means: how many degrees are there between 12 below and 7 above. There are 12 to take you from 12 below to freezing; and another 7 to get from zero to 7 above, making a gap of 19 degrees.