Find which chord is the root. That means which chord seems to be the one which you'll end the sond with.
Then starting on the root of this chord, you have to play the myxolydian scale. If the root is A:
-----------------------------7-9-10----
----------------------7-8-10-----------
-----------------6-7-9-----------------
-----------5-7-9-----------------------
------5-7-9----------------------------
-5-7-9---------------------------------
Modes help you sounding different, you can find where they come from in the tricks section of this site.