You may find that police officers in Guatemala are, at times, somewhat unhelpful. Generally speaking, the less you have to do with the law, the better.
Whatever you do, don't get involved in any way with illegal drugs – even if the locals seem to do so freely. As a foreigner you are at a distinct disadvantage and you may be set up by others. Drug laws in Guatemala are strict, and though enforcement may be uneven, penalties are severe. If you do get caught doing something you shouldn't, your best line of defense is to apologize, stay calm and proceed from there.
While many commentators claim that corruption is rife in Guatemala, don't take this to mean you can buy your way out of any situation. If it does seem that you can 'make everything go away' by handing over some cash, proceed cautiously and tactfully.
Reporting a crime is always a toss-up in Guatemala. If you're the victim of something really serious, of course you should take it to the police – the phrase you're looking for here is 'Yo quisiera denunciar un crimen' (I'd like to report a crime). If you've been robbed, get a statement filed so you can show your insurance company.
If it's a minor thing, on the other hand, you might want to decide whether or not it's really worth your while reporting it to the police.
Outside of those areas (and normal office hours) your best bet is to call Proatur, which operates a 24-hour nationwide toll-free hotline in English and Spanish. It can give you information and assistance, help deal with the police and even arrange a lawyer if need be.