Two important points about using a semicolon:
1. The two parts of the sentence on both sides of the semicolon make sense as individual sentences
2. The two parts of the sentence are both on the same topic and are closely related.
So in this example sentence:
I hate sunbathing; I get burnt really easily.
the use of the semicolon is correct because the two parts of the sentence could be independent sentences, and they are very closely related.
However, this example sentence using a semicolon is NOT correct:
I hate sunbathing; really boring and dangerous.
because the second part of the sentence would not make a sentence on its own.
Similarly, this example sentence is NOT correct:
I hate sunbathing; I drive to work every day.
because although the two parts of the sentence would make sense on their own, they are not closely related.
Got it? Now, here is a great online exercise in which you can test yourself! This page from Bristol University is excellent and challenging. It’s here.
You should read my other page on semicolons too!