This is described on the first page of Treasure Island. Jim Hawkins is telling the story as a flashback, because the old seaman's entry is the first conflict in the story.
The seaman, whose name is later given as Billy Bones, prefers to be referred to anonymously as "captain", and is haggard, dirty and seemingly of mercurial moods, but generally more foreboding than anything else. He is described as tall, strong, and "nut-brown", probably meaning that he is heavily tanned rather than brown-skinned by heritage, and scarred on his hands and most distinctly on his face, by a saber (a type of sword). He calls himself a simple man, drinking rum "like a connoisseur" and asking only for rum, eggs, bacon and a place to watch the ships from, and spends most of his time silent, though he gets more raucous when he's drunk. Despite his appearance, he is said to have none of the manner of a ship's crewman, but instead appears to be someone accustomed to either having his orders obeyed, or striking the person who hesitates or questions him.
Jim finds him to be much less intimidating than the others, perhaps out of naivete.