In terms of the IELTS speaking and writing test your lexical resource (vocabulary) accounts for 25% of your score. Therefore, part of your preparation for the exam should be to improve your range of vocabulary. Idiomatic expressions are extremely important for the speaking exam I suggest you learn some and get into the habit of using them as this is important to get to seven and above for that lexical resource. In order to achieve this I suggest you learn about 20 idiomatic expressions can be used for a wide range of common topics. Try to learn a few well rather than focusing on learning as many as you can. This is because this approach often results in not being able to use them correctly or forgetting to use them.
For the writing test, start by learning vocabulary under common topic areas. Some of the most useful areas are as follows:
6. The Environment
It may be helpful to keep a vocabulary notebook and write down new words that you learn along with sentences that use these words so that you can remember the context to use these words in. The key point about learning vocabulary is to make sure that you revise it and use it. Trying to memorise hundreds of words without revising them or practising using them will result in poor retention of these words and you will be unable to remember them when it comes to the exam.