Fact vs. Opinion:
To begin, you will need to know the difference between fact and opinion. Distinguishing between fact and opinion is an important part of any type of writing. In expository writing (writing to explain) you do not use opinions. In persuasive writing, you use opinions supported by facts.
View the video below for an overview of this distinction. The video was created for English Language Learners, but it gives a very effective overview of the topic.
Next, read the article "Distinguishing Between Fact and Opinion." Try the practice exercises at the end of the document.