I am guessing "Diff" and"Points" are two different columns in a table.
$result = mysql_query ("SELECT * FROM Mytable WHERE CompID = '2' and SeasonID = '13' ORDER BY Points ASC, Diff ASC");
The priority is given based on the order you place them. So if you want it all rows sorted by "Points" and then "Points" sorted by the "Diff", the "Points" needs to be first in the written order.
Also lowest (1) to highest (100) is an ascending order and highest (100) to lowest (1) is descending.
EDIT
Someone beat me to it.
Yea what he said.