In Excel, to enter a value in a Cell you do it by writing your entry in the input line.
To do this, first select (with the mouse by clicking into it or by moving with the keyboard arrows) the cell you want to write your formula in. Then start typing. What you write will be displayed in the input line as well as in the cell.
You can then hit ENTER or click on the little green icon (tick) if you are satisfied or the red cross if your want to cancel your entry.
Tip: You will see with time that using the keyboard sometimes can be faster than using the mouse and keyboard together (taking the mouse, selecting a cell, releasing the mouse and going to the keyboard and starting to type is very time consuming). The mouse is useful for the menus but for moving around the spreadsheet, the keyboard can be much faster.