It really depends on where the tightness is, assuming you're not one of those poor souls whose whole body seizes up when they try to lift their knee.
For stretching, in general, you'll want to hold every stretch a minimum of one full minute; the longer the better. It takes at least 30 seconds of a fairly intense stretch for the muscle to overcome its own stretch reflex. This is a defense mechanism where a muscle under stress flexes to prevent tearing. Great for when you slip on ice, but bad when you're trying to increase flexibility.
Also, stretching should hurt like hell, but there's a line you don't want to cross. Never bounce into a stretch, but slowly pull/push yourself into it. Breath slowly, taking a deep breath and steadily exhaling as you go deeper into the stretch. No matter what kind of pain you're in, your breathing should be like you're sleeping/watching TV/passed out drunk.
It's important to warm up before a deep stretch, but even more important is that you don't do major stretching before any kind of intense training, whether it's weights, kickboxing, or sprinting. Stretching temporarily (about an hour) weakens your muscles and hinders the stretch reflex, leaving you with bad technique, less power, higher susceptibility to injury, etc. The best time to stretch is after your workouts, when your muscles are already exhausted. Not only does this make them easier to stretch out, the stretching helps work out the lactic acid and toxic buildup that comes with heavy exercise.
For your kicking problem:
It's best to do these in order, but it's not the end of the world if you don't.
For the groin/adductors (inner thigh), the butterfly stretch is good. Sit down, put the soles of your feet together and use your hands to pull your heels toward your junk. From there, use your hands/elbows to leverage your knees toward the ground. You can also use your abductors (outer thigh) to do this, which is good for strengthening the muscles that initiate the kick. You can also grab your feet and pull your chest toward them, keeping your back straight.
A similar stretch is the frog, where you basically squat down like a frog, spreading your bent knees as wide as possible and pushing your butt back. This will work the same area as the butterfly, but with a wider range of motion.
The good old-fashioned one-legged hamstring stretch is good, too. Just be sure to lock the leg and keep the back straight, even though it means you only go about half as far. The hamstrings usually take a couple of rounds of sustained stretches before they really loosen up. On the second/third round, bend your non-stretching leg to the outside, and flatten the top of your foot to the floor. If you're into pain, you can also do the two leg stretch. I personally don't do that one. It makes me too angry.
After the hamstring stretch, pull your chest toward your rear knee (carefully). This will help loosen up your hips even more.
After that, try the quad stretch. Some people (gym teachers) like to do both legs at once, but I've found that I get a better stretch going one at at time, and it's easier on the back. On this one, fold one leg back behind you, so your heel is tucked to the side of, but not under, your butt. Then lay back, relaxing as much as possible. Keep your ankle flat, too, so you can get a good stretch for your lower shin and instep.
For the next one, grab your left/right foot and tuck it into your right/left elbow, and pull it toward your shoulder. This should be a pain in the butt, literally. As you get used to this, you can hook your knee inside the elbow of the same side arm and lock your hands, pulling your knee to your near shoulder and your foot to the far one.
The hip flexors can be a problem for a lot of people. These are the muscles at the very top of your quad, where it inserts into your hip. A good stretch for these is the lunge. Keep your shoulders square and your rear leg locked straight. Bend your front knee, but keep it vertically behind your toes. Straighten at the back until you feel the pull on your hip flexors, then find the position that makes you want to cry. Don't forget the other side.
The last stretch and probably the most important (for the average person) for the high kick is the straight split. Just slide your legs out to the side, heels up, putting your hands to the floor. This one hurts a lot, but there's a key to getting faster results. Once you've settled into your max, flex your adductors (inner thighs) so you raise up higher. Slowly relax the muscles as you exhale. Do this 4 to 5 times before finishing the stretch.
One dynamic stretch I'd recommend is the Tae Kwon Do axe kick. It's good for strengthening your hip flexors and obliques, but it's also one of the best ways to get flexible hamstrings and sinovial fluid into your hips (lubricates your joints). Swing the locked leg straight up as high as you can (after warming up with lighter versions of the kick) and chop it down. Be sure to keep your back straight or your heel will meet the floor and that will ruin your month.
If you do these stretches 3 to 4 times a week, you'll be really sore, but you'll also see a big change in your flexibility, which will allow you to focus more energy into your strikes (and less on keeping your balance). You'll have more control over your body, which will allow you to improve your technique, speed, and timing. Also, you'll be less susceptible to injury of the muscles and the joints. All good things.