Building your vocabulary is key to comprehension. In general, you will expected to define words according to their context and part of speech.
Online Dictionaries
There are many options available, but using a search engine to look up a word's definition may not be the best way to go. The definitions themselves may be too complex to be useful. These dictionaries feature student-friendly definitions:
https://www.merriam-webster.com/
Etymology
Knowing the etymology or origin of a word can be helpful. Most dictionaries will provide you with information about the date the word entered the English language, the original language it came from and other pertinent information; however, this is an additional resource: