Go through with IELTS Cambridge books -5-6-7. They will give you perfect preparation!
Apart from this, read science magazines and explore good newspapers.
Try to read fast , dont read sentence by sentence, and try to get the idea of each passage.
you can use pen in order to underline key info on your question paper in real exam while reading...
you will get better only with practice.
AS for writing,
Task1: follow-intro-body-conclusion format
develop atleast three ideas, each idea in each paragraph and back it up with solid example.
use connectors/discourse markers: in contrast, on the contrary, in addition, Therefore...etc
use a variety of vocabulary, dont use same word all the time, when u use a word exhausting, dont use it again, use it tiring, etc
Task2: learn specialized vocabulary terms for describing graphs. like sharp decline, nosedive, stable, unchanged, skyrocket...etc
Don't give your opinion for this type of writing. Just write down whatever key info is there.