A good way to start a paragraph is with a short, simple sentence that introduces the main idea of the paragraph. Teachers often call this a ‘topic sentence’.
I normally write my topic sentences by thinking about how many points I want to mention in the paragraph:
1. If I only have one point or idea, I usually state it straight away:
In my opinion, junk food is the main cause of childhood obesity.
(then explain this opinion and give examples in the rest of the paragraph)
2. If I have two or three points or ideas, I don’t usually mention them directly in the topic sentence:
There are two main causes (or ‘several causes’) of childhood obesity.
(then explain using “firstly, secondly...” or something similar)
Have a look through the essays that you’ve written in the past, and compare them with some of mine. Did you begin your main paragraphs with good topic sentences?