In Android Netrunner anything with "prevent" or "avoid" is triggered outside the normal timing rules as the thing it wants to prevent or avoid is about to happen.
This means whenever you are about to take net damage for the first time in a turn, you can pay 1C to activate net shield and prevent 1 of that damage.
From the rulebook (PDF) page 21:
Prevent or Avoid
Some card abilities use the words “prevent” or
“avoid.” Prevent or avoid effects are the only effects which can
disrupt another effect. A prevent or avoid effect states what it is
preventing or avoiding, and an effect that is prevented or avoided is
not resolved. Prevent or avoid effects can be triggered whenever the
effect they are preventing or avoiding is resolving.