Ask your tutor or someone at your uni for a copy of their guidelines, you should be given a copy so that you know what you can/can't do.
Generally speaking though, ALWAYS, reference things. That way you can show that you knew it was someone elses work and referenced it and didnt try and pass it off as your own. If you are referring to someone elses ideas/theories etc then re-word it and then reference it. If you copy it word for word then you need to do it as a quotation use speech marks ("......"), you still need to reference it as well. Generally though try not to quote large chunks of work - its not good practice. Check out your departments guidelines for how they want it to be referenced, different people want it differently!
Also, be careful not to just learn model answers etc for exams - this is cheating as well! If you are refering to someone in an exam answer, you still need to reference it.
When you work with other students be careful that they don't copy your work - its not just you copying them. Also, esp with calculations etc, you may all have similar data but different people can work things out differently, it will look funny if everyone worked their answers out exactly the same!
One of my friends at uni got caught for plagiarism last term. She was in a lab group with another person, he did his calculations the same as she did, so both got the same ones right and the same ones wrong. She didnt realise that he had copied her working until they both got called to a meeting with the head of school. The other guy told her then what he had done. They went to the meeting, the other person said that it had been him who copied. They both ended up with the same punishment though. They said that she was equally in the wrong as she shouldnt have lent the other guy her work in the first place. They both had that module invalidated, so are 10 credits down in effect. Seemed very harsh that she had only lent him her work to look at and he copied, and they both got the same in the end.
Anyway, moral of the story - be very careful lending people your work. Be careful about working with other people etc. If in doubt check with a lecturer or your tutor, so that you know that what you do/have done will be ok.