Why is there a lone pair in thionyl fluoride?
When you want to write a Lewis structure, I suggest you start from considering how many valence electrons each atom has. In your case, you would have:

S = 6
F = 7; total = 14
O = 6  

The final total would be 14 + 6 + 6 = 26. The structure you guessed for is correct, sulfur is the central atom. You start by drawing a single bond per each atom, getting something like this:

F
|
S–O
|
F


Then, you can draw a double bond for oxygen:

F
|
S=O
|
F


If you do the math, by counting the electrons that you put in the previous structure, you would have 2 + 2 + 4 (two S-F bonds and a S=O) = 8.
Then we will add in the lone pairs: three on each halogen, two for oxygen, and one for sulfur. Therefore we add 3 &times; 2 &times; 2 = 12 electrons (three lone pairs on each fluorine) plus 4 electrons on O (the two lone pairs), and we have a total of 24. The final 2 , adding up to 26, come from the lone pair on sulfur. 

This makes sense because sulfur is in period 3, so it is possible for it to have more than 8 electrons: in other words, the octet rule applies not only for 8 electrons, if that makes sense.