First, you need to locate the cell into which you wish to enter the data.  Click on the cell.  The active cell is indicated by a dark border round it.

Notice the cell address (D3) in the address box (top left).  Also notice that the "D" and the "3" column and row headings are also highlighted to help you see the cell address.

You are now ready to enter your data into the cell.  As you do so, the contents of the cell also appear in the formula bar.  When you have finished entering your data, hit (1) the ENTER key, (2) or the TAB key (3) or any of the ARROW keys (4) or the TICK button or even (5) simply click in another cell.  ALL of these methods tell Excel to accept the data you have entered.  Remember, you can also click the "X" button in the formula toolbar to cancel the entry, or simply delete the contents of the cell later by selecting the cell, and hitting the DELETE button on your keyboard.