This picture shows text in cell A1 will text wrap turned off, and text in cell A3 with text wrap turned on.

To wrap text in a cell:
1. Right click the cell and choose Format Cells
2. Select the Alignment tab
3. Place a check mark in the box next to Wrap Text and click OK

To set the wrap text property for the entire sheet, select one cell and use the shortcut Ctrl and A to select all the cells, then proceed to format the cells with wrap text.