To add cells you must first click on the cell where you want the total to be, in this case it would be G7.
Enter equals then sum to allow excel to know that you want to add up cells, then use an open bracket before entering the selected cells.
The highlight over the cells that you want to add up, this will be shown by a dotted lime around the selected cells.
Then use a close bracket to finish off the formulae. After you press the close bracket just press enter and the addition will be done.
The creator of this guide has not included tools