Addition in excel is straight forward, autosum is the easiest function to use when data is continous. similary sum function can be used. when data to be added is in different non continous cells then you may have to use + sighn or a nested sum function.
Excel is powerful in data analysis, you can even add data in different columns and different columns altogethter. Remember you can also format the autput cell, like to change the colour if a given criteria is met. Apart from addition excel excel can perfom other calculations like divide and multiplication.
The example below will give us an insight on how to do sum in excel. It is a straigh forward example with illustrations that are clear to understand.

Autosum function
click an empty cell below the numbers to be summed, On the main menu(home) click the auto symbol, (circled in red in the image below)
Then enter from the key board.
The formula is also displayed in the formula bar
The sum will be done.

Sum different cells not following each other
You can do additions using cells that are not continuous eg B5 + B7 + B10
Simply on the cell where you need your answer to be type =then click B5 the + sign from keyboard then click B7 followed by +sign from keyboard and then click B10
and finally enter from keyboard

Sum in different rows and columns
You can use auto sum as follows
Click the cell where you want the sum to be and then click auto sum from main menu(Home)
now change the range to the one required, ie change C5 to B5
finally enter from key board
The same can be accomplished using + as in adding non continuous cells.