Deleting data is also easy. Just highlight the cell or cells with the data you want to delete, and then press the Delete key. You'll be asked exactly what kind of data you want to remove, and if you want to remove formatting as well or to leave it in place.
If you want to totally eradicate the cell along with its contents, right-click it and select Delete. This will cause the data to the sides of the cell to move in to fill the space.
To insert a new cell, right-click where you would like to it to appear and select Insert. The data in the cells surrounding the inserted cell will shift to make space for it.