Use a comma before a coordinator: I researched the topic, and I created the presentation.

When the dependent clause is at the beginning of the sentence, it should be followed by a comma: After I researched the topic, I created the presentation.

Do not use a comma when the dependent clause is at the end of the sentence: I created the presentation after I researched the topic.

When the transition begins the sentence, it should be followed by a comma: First, I researched the topic. Then, I created the presentation.

When the transition connects two independent clauses, a semi-colon comes before it and a comma follows it: I researched the topic; afterwards, I created the presentation.