In IELTS Speaking Part 1, the examiner is mainly testing you for two things: i) everyday vocabulary and, ii) basic grammar.
For almost every answer, you should give a two-part answer: First, a direct answer to the question and then add more information.
A 'direct answer' does not mean the same as 'an immediate answer' – a 'direct answer' means 'not an indirect answer'. Indirect answers lower your coherence score. Here's an example of an indirect answer:
Question: "Do you work or are you a student?"
Answer: "Oh, I've been working for five years."
That answer does tell the examiner that you are working but it does so indirectly. The question asks you to choose between "A" and "B" and, in order to answer directly, you should first choose between A and B and then add some suitable extra information. So, a more suitable answer is, "I'm working. In fact, I've been working for five years."
There are times in the Speaking test when it is suitable not to give an immediate answer, for example, when it's a difficult question and you need to think for a second before answering or when you want to make a comment about the question. This is natural even when you are speaking your native language. In the case where you need time to think, you should tell the examiner that you need a second to consider the question; don't sit there silently thinking about your answer.
In other words, you should always start speaking immediately after the examiner asks a question but what you say does not always have to immediately be the answer to the question from the moment you start speaking. However, you should not need to speak this way very much in Part 1 because almost all questions in Part 1 are rather simple, information-seeking questions. This advice is more suitable for some Part 3 questions.
Be very willing to give the examiner a lot of information. ("Information" includes your feelings and opinions.)
The best way (but not the only way) to give new information is to include the information in the same sentence as your direct answer. When you do this, you usually use a relative pronoun such as, "which" to make the whole sentence into a complex sentence.
When you give extra information, make sure it is new information, not information that everybody already knows.
Here's an example of extra information that is not new information: "I study chemistry, which has a lot of experiments." Almost everybody knows that the study of chemistry (usually) includes experiments. That's almost like saying, "I study history, which is the story of the past." But if you say, "I study chemistry, which has a lot of interesting experiments" then you are giving new information to the examiner – the information that the experiments are interesting to you.
Note that there are times when it is not suitable or natural to use a relative pronoun to connect the extra information to your basic answer. For example, if you say, "I'm working, which (is something) I've been doing for five years" then your answer will sound a little unnatural and forced.