You have to have the following qualities to become a good tester:
1. Constructive Critic: to test the product with a different perspective
2. Analytical: to analyze all aspects of product to be tested
3. Medium Technical Knowledge: to write queries
4. Good Business Process understanding: to know or understand business process and functioning
5. Good Reporter: for reporting bugs in a manner that it is easily understood by developers.
6. Think yourself as customer: to examine the product as per customer requirements
7. Understand Scope of testing very clearly: there are many aspects of testing – performance, functional … which one is under your preview, has to be very clear
8. Documents: while handover of the product from development to QA for testing you must scan each and every document to understand customer specifications and requirement very clearly
9. Capture and write in detail: first you have to prepare test plan then test cases and then in your test report you have to be very clear and descriptive to explain the test scenario with enough number of screen shots
10. Good Communicator: you should be able to simulate and explain the bugs to developers if required.
11. Patience: to be a good tester this is the foremost requirement, you can not test a product in hurry.