I'm hoping your instructor gives you basic stretching exercises to do in class as part of your warmup, so just focus on those and do them every day. 5 minutes stretching every day will result in better flexibility than half an hour's stretching once a week. When you do any stretching at home, here are some key points to remember:
1. warm-up your body first. trying to stretch cold muscles can cause injuries. Think of your muscles like an elastic band. If you take an ice-cold elastic band and try to stretch it, it will quickly break. Take that same elastic band and gently warm it up and you can stretch it a lot further before it breaks. Your muscles are the same. So, jump up and down, run around a bit or just swing your legs and hips before you start stretching. Then in between each stretch, jump around a bit to stop your muscles from cooling down.
2. Breath into the stretch. A lot of people hold their breath while doing a difficult stretch as a natural result of the stress (and possible pain) they are putting their body in. This causes your body to tense up making it harder to stretch out properly. By doing deep breaths through the stretches your body will relax allowing you to stretch further for longer.
3. Don't hold a stretch for too long. Holding a stretch for too long can also result in injury. Going back to our rubber band, if you hold it at the maximum stretched position for a long time, even if you don't increase the stretch, it will break. instead, hold the stretch for 5-10 seconds and then let your muscles relax. If you want to keep stretching, stretch a different group of muscles while you wait for the others to recover.
4. Hold the stretch, but don't bounce it. When you are stretching you want to go to your comfortable limit (it should be uncomfortable but not painful) and hold it in that position, rather than bouncing to try and increase the stretch. Go slowly and listen to your body. It will tell you when it's ready to go deeper. What you can do is start off a stretch in a comfortable position, hold it and then relax. Then go back to the same stretch and go a little deeper.
Hopefully this helps. One final piece of advice I will give is be patient. There will be days when you can't stretch as far or as deep, and this can be frustrating. Don't try and force it and over time your body will become more flexible.