With the popularity of taekwondo these days, and the bastardization that goes along with it, the traditional Korean dobok has taken a lot of punishment.
At one school I trained at, it's been completely replaced by a T-shirt sporting the place's name and logo, and the kind of dobok-like gym pants you see in martial art product catalogs. Both of these articles of clothing are, last I checked, black with red trim. This is not just the students' standard garb, either: black belts, instructors, and even the school's owner wear this every day. (This is one of the reasons I don't train there anymore...they also don't hang the South Korean and American flags at the head of the dojang anymore. Sigh.)
In a perfect world, all students of taekwondo still wear the traditional dobok. A set of white pants with a stretchy waistband for the legs; a double-breasted (or possibly flat-front), long-sleeved white shirt (with a white collar, for colored belts) on the top. One's belt is the correct size (it should hang to just above the knees) and is tied properly, with even lengths of belt hanging from each side of the knot.
Tying your belt is not very difficult. It probably won't come easily at first, and your first couple attempts at tying it won't come out looking exactly like your instructor's belt (especially if yours is brand new), but after a few weeks of careful practice you'll be tying it right without even thinking about it.
(To tie your belt, start by holding the center of the belt just below your belly button. (If your belt has any stripes on it, put the striped end on your right.) Wrap the left end around you, and hold it behind the belt's center; then wrap the right end around you, holding it in front of the belt's center. Fold the right end down, then up behind the center, and back to roughly the same position it was just in. Make sure the belt is snug (but not tight; you want to be able to move), and you may want to jog it an inch to the right so the ends come out even. Now make an overhand knot, with the right end over the left end, and give the ends a yank to tighten it — you should end up wth a knot that somewhat resembles a fortune cookie (with the two points pointing towards your right). Check the ends of the belt — are they about the same length? Your stripes should be on your left now. If the belt ends are sticking off at weird angles, loosen the knot slightly and right where the ends emerge from the knot, fold them down. If you have text on your belt, it should be readable as someone faces you.)
A dobok is to be respected. When you take it off, don't crumple it up and toss it in your gym bag next to your sweaty kyorugi gear — fold it neatly and carry it. Never, ever throw your belt on the ground: your belt is a symbol of all the work you've put into your study of martial arts, and is to be respected as such. The longer you've been studying, the more respect it demands. When your belt is not around your waist, it is folded neatly and placed on top of your dobok — do not leave it on the floor. You should not wash your belt either (the reason behind this can be found here).
Don't wear your dobok outside of the dojang. It should only get dirty from your sweat, not from rolling around in your back yard. Never eat in it, and don't show up at your dojang wearing a dobok with a big ketchup stain on it. Don't let it develop pit stains (washing it right after you get out of it is a good way to prevent this, and bleach it it you have to). Keep it crumple-free — you don't have to get it pressed (although it's nice every now and then), but hang it up when it comes out of the dryer.
If you're new to the study of martial arts, your instructors will be impressed with the care and respect you show towards your appearance and your dobok. They will recognize that you are serious about your practice, and come to respect you as a student and individual. The longer you study taekwondo, the more this respect towards one's dobok will be expected — what would you think if your senior instructor showed up to class wearing a ratty, yellow-under-the-armpits uniform? Don't underestimate the importance of your appearance.
Good doboks are expensive. Don't treat it like you would treat the gym clothes you'd wear at in a public high school.