This is a very personal question. You are an individual, so certain topics hold more interest to you than others. Look to your hobbies and other interests to get subjects to study in English.
Once you have found an interesting topic to explore in English, you can more easily find an interesting and useful original source material.
Source material could include:
To view your choice of the above, click the topic area on the Table of Contents panel. A short message about the topic will appear in this panel.