This is the best place to start! From this page, you can find out what the CEFR Levels are and what they mean. You can also take the level tests below to see what your current CEFR English level is. There is a grammar test, a vocabulary test and a reading test.
The CEFR levels range from Level A1 (Beginner) to Level C2 (Proficient). Knowing where you are on the CEFR scale can really help you know what you need study and how to improve. To get a good result in an English test like IELTS, you will need a CEFR level of at least B2 (Intermediate), so you the resources on this page to see what your level is.