The top part is the menu (of course you know this).
Underneath where the E7 is. This is the name of the cell you have the cursor in. (you will see later that you can give this cell a name and then recall this name).
On the right side of the E7, after the =, the blank part is where you enter your value and formulaes (you will see later...).
On the bottom, you can see the wording Sheet1, sheet2, sheet3. These are 3 different sheets you can use to enter your work. You can add sheets (in menu insert) or rename them (by double clicking on the tab).