Do I set my flash to TTL or manual?

Both.

TTL (fully automatic flash measuring) is a revolution, and you should use it whenever:

• The subject moves
• You move
• You have little time
• You use one flash on camera
• You use multiple flashes but they are within eyesight of each other

Use it as is (your flash shows “TTL” on the back display), and do not forget to use Flash compensation to adjust to taste (or when shooting dark or light subjects).

I used TTL here, Sunday night (with -2 stops flash compensation, or it would have been too bright):

Berlin Nightclub in Oakville

Manual (your flash displays “M” at the back and you set the power level to 1/1, or 1/2, 1/4, etc) is useful in cases almost opposite to the previous, namely:

• You and your subject are stationary
• Things are predictable and you want full control
• You have time to meter, try, and iterate
• You use multiple flashes and they cannot see each other
• You use Pocketwizards

You can even mix: use TTL for most flashes but fire small accent lights using Pocketwizards and manually set flashes.

Earlier that same Sunday night, when I had time, I used three speedlites set to manual and fired by Pocketwizards, here:

Berlin Nightclub in Oakville

You can probably see I also used some Honl gels: one red and one purple. And one flash was zoomed in to light the picture.

A good photographer knows both TTL and manual. Practice with both, and make them “your own”.