sort_values

df.sort_values(by='col1', ascending=False)

# Pandas for Python

df.sort_values(by='col1',asending=True)
# OR
df['col1'].sort_values()

>>> df.sort_values(by=['col1'])
    col1 col2 col3
0   A    2    0
1   A    1    1
2   B    9    9
5   C    4    3
4   D    7    2
3   NaN  8    4