It's easy to get too worked up about details like the lengths of your sleeves, and the internet is full of forums where you can obsess endlessly about a few millimetres here or there. It's really not terribly important compared to the basics of proper shoes, a decent fit around the coat shoulders, and a well-tied tie. Still, it's generally accepted that it is more stylish to show a half-inch or so of shirt cuffs and doing so will mark you out as someone who has taken a bit of extra care in getting a good fit. Too many people unsure about the correct fit will err on the long side, and end up with sleeves that end part-way down their hand, which does little for your general appearance.
There's plenty of room for personal taste in where, exactly, your sleeves do end, but somewhere around the wrist-bone is traditional, and well before the top of the hand is best. Whatever length the coat-sleeves are, your shirt-sleeves should be about half an inch longer. This helps to frame the jacket, and also means your cufflinks will occasionally show from underneath your suit.