Perhaps you could try doing practice writing? Maybe write journal or diary entries in English to continue practising English and using new words. If there are words you do not know, maybe write them in new sentences to test whether you know their meaning and the relevant grammar. I hope this helps. Obviously listening/reading English sources is also very beneficial as mac said.

Hi Xom, how are you doing? Well, in my case I have a lot of problems in Writing. Everyone that I talked told me to read newspapers (might be online), magazines with subjects that I like and always put English subtitles on the movies I watch. You could subscribe http://www.theguardian.com/international for example, it is free. Podcasts are amazing especially for Listening, and I recommend http://teacherluke.co.uk/the-podcast/archive-of-episodes-1-149/. Another one, http://www.engvid.com/ is also free and has classes in different levels. If you like to watch lectures, https://www.ted.com/. Good luck!

Concentrate on your weaknesses. Was this a practice exam? Do you have it to refer to? Look at one of the places you made a mistake. Write a short notebook entry here in the Notebook section, just a few sentences, that uses the same words or grammar correctly.