Before you set off in search of supplements, first of all ask yourself if there's a way of sorting out your problems or niggles by leading a healthier lifestyle (improving your diet, sleep or exercise). Supplements aren't just a permanent weapon you can use to redress imbalances caused by an unhealthy lifestyle or bad habits.
If supplements could still help you, head down to your local health food store, pharmacy or supermarket. Ideally, ask for advice, and at the very least make sure you scritinise the labels in detail: they're required by law to be precise about what the supplement does, so you should be able to find the right one for your needs.
The best thing to do is to get a 3-month supply (depending on the instructions). Don't take more than the recommended dose and don't combine supplements containing the same nutrients: the risk of overdosing may be small but it is real. You can get too much of a good thing!