To enter a value into a cell, select the cell by tapping on it. Then click in the formula bar located at the top of the screen. The keyboard will appear, allowing you to enter the value that you wish to attribute to the cell. Once you have entered the value into the formula bar, press the Done key on the keyboard in order to transfer the value to the cell.
Alternatively, once you have entered a value into the formula bar, you can simply select another cell and the previously selected cell will be updated with the contents of the formula bar.
If the formula you wish to enter is long, it may be easier to enter the formula into a larger field. You can do this by double clicking on the cell, whose formula you wish to enter. Doing this will display an Edit Formula window, allowing you to see the entire formula as you type it.