What is more important: grammar or vocabulary?
Amal writes, "I want to take the IELTS exam in July. I do not have a lot of time. I need 6. Do you think I should learn grammar or words?"
I think you should learn both. David Wilkins writes, 'Without grammar very little can be conveyed, without vocabulary nothing can be conveyed' (Thornbury, 2002). If someone says something using the right words but the wrong tense, we are more likely to understand the message than if they use the wrong words. Ultimately though it is very difficult to separate grammar and vocabulary, you need both to communicate effectively. Some linguists talk about words having their own 'delicate grammar' (lexicogrammar).
The IELTS exam tests your skills in speaking, listening, reading and writing. It does not test discreet grammatical items on their own. So, practising these skills will help. If you are going to study grammatical structures, try to use them in context, in your speaking and writing. There are some good vocabulary books you can get to help prepare (e.g. Vocabulary for IELTS), you can try some practice tests, and you should probably also find yourself a good tutor.