Format cell window
Select single cell or range of cells and click a toolbar button or use menu option Menu->Format->Cell format. This will show a window with 7 possible format types. Select your preferred one. Depending on the type, there are some additional options available. Click OK.
General is a default setting for all cells. It is not specified whether a value in cell is a number or text or any other type. By default number is aligned right and text is aligned left. You can change this setting by selecting toolbar align buttons .
Both Custom Number and Custom Text are for number formatting, but there is a slight difference how a number is interpreted.
Additionally to the formatting described above, there are also options available in the toolbar. They include font properties, cells merging, borders, font and cell background colours, alignment (horizontal and vertical), text wrapping.
Most of the formatting options available in the toolbar are also available from the menu (Menu->Format->[select option]).