You can select a single cell by tapping on the cell, and select an entire row or column by tapping on the relevant row or column header.
To select a range of cells, tap on the cell that defines one corner of the rectangular range of cells that you wish to select, then press and hold down on the cell that will define the opposite corner of the range. After one second the range will be selected, allowing you to perform any functions that involve a range of cells. Another way to select a range is by double tapping and on the second tap, hold down and drag to define the range.
If you wish to select several rows or columns, select the first row or column (as appropriate) and then press and hold on the last row or column (as appropriate). After one second the range selection will be made.