There are three passages to read and 40 questions to answer in the IELTS exam. Start with Reading Section 1. The questions usually follow the order of the reading text so start with question 1 followed by question 2 etc. This will make it faster to find the part of the text that gives the answer to the question. Answering the questions out of order, say by choosing the 'easier' questions first, risks wasting precious time.
So Reading Section 1, then Section 2 and Section 3 last. Questions in the order of 1, 2 ,3, 4, 5 --------through to 40!