Do I Need To Prepare In Advance?
Most definitely, yes, you really do need to prepare in advance. If you are recording your podcast on a weekly basis, spend the week before thinking about your subject and jot down any notes as they spring to mind.
Even if you are spectacularly good at winging it and making stuff up on the spot, you will still benefit from having a simple list to follow, so that you don't accidentally miss something out or overlook an important item.
Writing a Script
Aim to end up with a basic script, so that you know what you are going to talk about and can be confident that you won't miss anything out. However, do be careful not to make any noise shuffling your paperwork, as that might be picked up by the microphone.
This is particularly so if you are using a condenser microphone rather than a dynamic microphone, as the former are more prone to picking up even the slightest of background noises. If you have a laptop or iPad to hand (other tablet computers are available), then maybe you can just read your notes from the screen and scroll up and down as required, rather than shuffle your paper notes.
Research Your Topic
During your free time, make sure that you keep an eye on the news in case anything relevant is mentioned. It may be worth subscribing to online newspapers or even listening to other like-minded podcasts, so that you are fully up to speed with what is going on in the world relating to your chosen topic angle.
If your podcast is about entertainment and celebrity news, then buying a few of the latest teen magazines each week will certainly be advisable, and if you are concentrating on the theme of fitness and sport, then keeping abreast of the latest ball games is obviously a must. However, podcast research may not always be quite that straightforward and simplistic, and it may need more of a hands-on approach, where you try out your ideas before you tell people about them, such as DIY projects or baking recipes.