To select a cell range using mouse:
1. Click, and drag to select the cells you want.
To select a cell range using keyboard:
1. Place the cursor on the desired cell and press Shift + Arrow keys.
2. To select all the filled cells in the data range from the current cell, press Cmd/Ctrl + Shift + Arrow keys.
3. To select the current data table, press Cmd/Ctrl + A.
4. To select the entire sheet, press Cmd/Ctrl + A.

Cmd - For Mac users
Ctrl - For Windows users