It used to be a French diminutive, sort of childish or young, for Martin (now rarely ever heard, because the character is so popular it displaced this use) in the vein of Billy for William, but in the albums Tintin is not a Martin, it's his real name.

And it has strictly nothing to do with the english word tin, of course (that's rather étain in French)

As for how the name should be pronounced, it's hopeless enough to get it right that they have it pronouced like the word tin in English and have adapted it in other langagues too ("in" is rather a compounded nasal vowel in French without the "n" sound heard, a sound that doesn't exist in English and difficult for anglophones to pronounce perfectly, even those fluent in French.)