Before you get started with your show you need to have a strong, unique idea and a format that makes your show stand out. How can you be original?
Try to avoid the usual formats of either a 1-hour long guest interview or a "2 co-hosts talk about a topic". There are a lot of podcasts out there and you could get lost in a sea of interview shows if you don't come up with something fresh.
On the other hand, don't spend too much time stewing over this. If you're filling a gap in the market or your industry where an interview show doesn't exist, then go for it. You can take a regular format and put a little spin on it; say that all the shows in your niche are quite lengthy, maybe you can introduce a shorter format?
This is exactly what our in-house podcaster James did with his show, Indie Bites. He took an existing format "interviewing entrepreneurs about how and why they grew their businesses" and packed it into a 15-minute episode. He also tries to add a unique interview style and some extra elements people get to know the show for; such as his recommendations section.
Here is a list of formats to choose from, along with a relevant show for inspiration:
Interview - The Joe Rogan Experience
News - The Daily
Co-hosted - My First Million
Narrative - Homecoming
Documentary - Breaking Brand
Unique - Song Exploder
Podcasting is meant to be fun, so make sure you're making your show about something you really enjoy. Create a list of all your ideas for names and formats, then start to brainstorm to decide on your favourite.
When you're deciding on your format it might also be useful to think about your length and publishing frequency. Think about your audience, would they prefer a 2 hour long show, or a 2 minute one? Would they like to listen to the show once a week, or once a day?
(I will be posting Part 2 soon.
If you want to go through all the parts now, I'm posting the full guide in the comments.)