The basic parameters of a JC talk are:
Duration. Plan to talk for about 20 minutes and spend an additional 5 minutes fielding questions.
Topic. Typical journal club talks discuss an academic paper which is not the speaker's own work.
Preparation. If you're a less-experienced presenter, the organizers can help connect you with a subject-area expert who can spend a little time helping you get oriented with your topic.
PowerPoint. Feel free to use it if you want. In the interests of saving preparation time and practicing extemporaneous speaking, you may wish to keep your usage of it to a minimum, only to show key plots or equations.
Feedback. If there's any particular kind of feedback you'd like on your talk, let the organizers know, and they'll try to make sure you get it.
Here are some ways you could choose a topic to present:
It's important that Journal Club appeal to as broad a cross-section of the department as possible, so try to choose topics which will be accessible and interesting to those who are nonexperts in them.