Plagiarism's a problem.
You can lose marks for it. You can even lose all your marks for it - it's quite common for tutors to give a mark of zero for a piece of work if they think it's plagiarised.
How can you avoid plagiarism?
You'll find everything you need right here. There's a page on what plagiarism is. Then there's how to use other people's work in the right way, and not plagiarise it. Then there are some examples of students getting this right, and some of students getting it wrong. At the end, there's some good advice about what to do when you're planning your assignment. That's the best place to avoid plagiarism.