What should I do if I was burned by scalding tea from a restaurant?

First, only if the tea was hotter than tea is normally served might the restaurant be liable; if the tea was at the typical or common tea temperature, then 1) the restaurant was not careless, reckless, or otherwise did anything wrong; and 2) there is a known risk in drinking hot beverages, which you accepted by ordering tea.

If the tea was hotter than it is supposed to be served, however, the restaurant may be liable, or financially responsible for your injuries, because it may well be negligent, or careless, to serve tea at too-hot a temperature. But even in that case, it may not be worthwhile to take action: the restuarant would only be liable for:

1) your out-of-pocket (not paid by insurance) medical costs;

2) your lost wages, if any; and

3) IF you suffered fairly long lasting (weeks, at least) significant disability, disfigurement, or impairment of life functions, for some amount of "pain and suffering"--but the amount you could recover would only be significant with some long-term or permanent impairment or disfigurement. For a few days or a week or two of pain, or light scarring that fades, the amount of pain and suffering you could get is negligible at best.

Therefore, it is entirely possible that even if the restaurant was at fault, that the amount of money you could recover would not justify legal action.