You get more marks for using a good range of vocabulary. This means you should not repeat the same words lots of times but you should have a variety of words in your writing task.
Don’t always repeat the same words. Use synonyms or antonyms.
Use adverbs and adjectives to make your writing more interesting.
Think about the style of what you are writing and how formal or informal it is. Choose your vocabulary carefully depending on who the reader is.
Have a look at this text which uses a lot of descriptive language. Look at this blog post to see how to use informal language or this film review to focus on the vocabulary needed for writing reviews.
Do you think about using a good variety of words in writing exams?