One effective way to improve English language skills is to complete lessons and courses. One site offering free English learning content (reading, writing, listening, speaking), and certificates indicating your level of proficiency is https://www.worldenglishinstitute.org/ .

Completing intermediate, advanced, and supplemental courses will make you eligible to receive a Diploma. Additionally, completing the above courses will help you pass the TOEFL (Test of English as a Foreign Language) Exam (one of the criteria when seeking admission to UK/USA/Australia Higher Education courses).