Knowing how and when to cite is tricky, but the rules for a speech are very similar to the rules for a paper. 

If you have a slideshow or other accompanying visual, remember to include at least a works cited list at the end.  If you are using pictures, those should be given some kind of attribution, too.  Typically, a website url at the bottom of the photo will do.  Ask your instructor if you aren't sure. 

• Mention your source in your speech whenever you:
• Use direct quotations Paraphrase (put someone else's ideas into your own words)
• Use an idea that someone else gave you in a conversation, email, class, etc.
• Describe an idea that influenced your work
• Want to use an expert opinion or lend authority to your own opinion
• Give any information that isn't common knowledge
• Use any or all of a previously created work of your own design (yes: self-plagiarism is a real thing)