Students often worry that they need to use "complex structures" in the speaking test. But what is a complex structure?
This website explains the difference between simple sentences, compound sentences and complex sentences. You'll notice that compound and complex sentences are much easier than they sound! I'm sure you use them all the time without realising it.
Here's my advice: stop worrying about the need for "complex" grammar. Instead, focus on expressing your ideas well. As you explain your ideas in detail, you will naturally produce longer sentences which contain a variety of grammatical features.