The difference in scale modes is the starting note. Take the F major scale, and note the notes:
1 - 2 - 3 - 4 - 5 - 6 - 7 - (1)
F - G - A - Bb - C - D - E - (F)
Now start with the fifth note, being C, and play it until the next C.
Which gives:
5 - 6 - 7 - 1 - 2 - 3 - 4 - (5)
C - D - E - F - G - A - Bb - (C)
That's a mixolydian scale, which has a structure of the standard scale, with the root note being positioned as the fifth, as the melody probably suggests. In reading, you can play along with the F major scale, and you'll sound right. Now if you mess up and want to keep with the tempo/band, just remember that you play a major fifth (C) at the beginning of the bar, not the major F the key signature would suggest.
Now, I am not an expert in modes, as I have been explained the mixolydian mode because of my interest in funk
I want to refer you to this page
The Libster , where, under "lessons", you will find a page explaining the why and the how of scale construction, if you are interested into a deeper understanding.