Side-effects: There are no studies showing serious side effects from creatine use. There are also no studies of long-term creatine use, so caveat emptor. However, it is a natural substance produced by the body, so as long as you use it in appropriate dosages, with on/off cycles, it is unlikely to cause any damage. Side effects include bloating and stomach cramps in a small number of people.
Dosage: Typical dosage is 3 - 5 grams daily. Although many people recommend a 'loading' phase when you first start using creatine, research shows that this is unnecessary - within 30 days you'll have the same results with or without the loading phase. Loading is quite simply a waste of money.
When to take it: Most people take creatine 30 to sixty minutes or so before a workout. Based on the research, this makes sense.
Cycling: You should not take creatine on a permanent basis. Most users 'cycle' their usage, for example two weeks on, one week off, or six weeks on, three weeks off. Whatever fits in with your training cycle (you do have a training cycle, don't you?)
Do I take it every day, or only on my workout days?
My understanding of how creatine works suggests that you don't need to take it on your off days. Many people do, so make your own choice.
Why doesn't it work for me?
There are some people for whom is doesn't seem to work. This is either because physiologically if doesn't work for them, or in many cases because they're not working hard enough. The bottom line is that unless you're pushing yourself really hard in the gym, taking creatine is pointless.
Creatine for teenagers?
Given the lack of long-term research, it is not recommended it for under-18s.