I'm 29 male and the last two days I've got Ulcer (canker ulcer) at back left moler? Only painful when eating what shall I do?

Suffer!! Sadly, there are no reliable treatments for apthous ulcers. Fortunately, they generally go away in 10-14 days. If you get frequent recurrences, if you are just miserable, or if the ulcer doesn't go away you may want to see your doctor for symptomatic relief and to make sure that the diagnosis is correct and that it is not something worse (unlikely, but not impossible).
Try Chloraseptic. you probably have a Canker Sore or Aphthous Ulcer. It will go away on its own but it can be painful when eating and drinking. Chloraseptic liquid will numb the pain. It is at your local pharmacy. Feel better.