Each cell in a spreadsheet can hold some bit of data. It might be a number, it might be your name, it might be a complex formula that keeps track of how much money you've spent on gasoline this year. In any case, you need to get that data into the cell before it's going to do you any good.
Entering data in a spreadsheet isn't like using a word processor. With a text editor, you just click the mouse where you want the cursor to appear, and then start typing things in at that point.
Putting stuff into spreadsheets is a little weird because although you click on the cell you want to put data into, you actually do your typing someplace else. That someplace else is called the editing box, and is usually located at the top of the screen.
Editing (changing) the contents of a spreadsheet is pretty much the same as entering new data. One option, in fact, is to just click on the cell you want to change and just start typing "over the top of" the existing data, wiping it out in the process.
A more genteel approach is to click once on the cell you want to edit, and then click up in the editing box and change whatever needs to be changed.
Fairly often, you'll have data in a cell that you just want to get rid of. You should click on the cell, press the delete key, and press Enter. Don't ever click on the cell and then press the space bar!!!. This looks as if it's doing the right thing because the cell goes blank, but there is "hidden" data in there -- that one space -- and that's enough to confuse some aspects of the program.
You can also delete the contents of a whole range of cells by selecting them with the mouse and choose Edit:Clear from the menus.