How do I improve my vocabulary?

I am looking to get into a really good boarding school, and to get in I have to take an ISEE-Independent School Entrance Exam. Half of the test is all about vocabulary, and I don't know that many words. I'm 13 and I'll be going into 8th grade after the summer. Are there any online workshops that help me learn vocabulary and test my knowledge? Also are there any ones that are free?