Start with a topic sentence. This is your main idea. State it.

Next you need, supporting details. These sentences, at least a few of them, give details, facts, and support to your main idea. Consider transition words to connect your sentences (eg. next, also, finally).

End with a closing statement. Restate your main idea in different words.

Here’s an example paragraph from another site.

There are three reasons why Canada is one of the best countries in the world. First, Canada has an excellent health care system. All Canadians have access to medical services at a reasonable price. Second, Canada has a high standard of education. Students are taught by well-trained teachers and are encouraged to continue studying at university. Finally, Canada’s cities are clean and efficiently managed. Canadian cities have many parks and lots of space for people to live. As a result, Canada is a desirable place to live.