There are two ways people usually go about choosing a topic. The first way is to choose something you really love -- something you're very passionate about. This can make a podcast that's really enjoyable to create as well as to listen to. The problem comes in when you decide that it might be a great idea to make some money with your podcast (whether to help pay for hosting or to use for pizza) and you find out the topic you've chosen is too small to really make any money.
The other way to choose a topic is to go to the other end of the spectrum, and to do some research on niches that make a lot of money. Some of the obvious topics are health, golf, mortgages, etc. There's money to be made in those niches. But there's a problem with deciding to do a podcast on mortgages if you're just in it for the money...
...after a few episodes you're likely to experience what's known in the industry as "podfade" -- that's when a podcast just... kind... of... goes... away.
Because even if the niche has the potential to make a lot of money, if it's not something you're interested in, it'll become a drag to create another episode.
So the best way to choose a topic is to go somewhere in the middle. Find a topic you're interested in, but which also has the potential to make some money. (Note: In just about any topic there's usually ways to monetize a podcast, but here I'm talking about the easiest and most sure route).
Let's say you really love collecting root beer bottle caps -- it's your passionate hobby. Starting a podcast with that theme is probably not the best idea -- at least, not if you want a good chance at monetizing your podcast in the future.
Instead, move more toward the middle of the road and maybe do a podcast for collectors of all kinds. Even though you collect root beer bottle caps and someone else collects agates, you both have a common passion -- collecting. You can now do a podcast that will appeal to collectors of all types -- you've broadened your market tremendously.
And on the other end of the scale, if you're starting your own podcast to monetize it right off the bat, choose a topic with money-making potential, but that is also of interest to you. For example, if you like to work around the house you could do a podcast about home improvement. And people who repair their homes are sometimes interested in home mortgages, so there's a way you can tie that big "money-maker" niche into a niche about which you're really passionate.
There's nothing wrong with starting a podcast that you never monetize -- a lot of people get into podcasting for the sheer fun of it and to connect with others who have similar interests. But if you are going to want to monetize your podcast, the time to think about that is at the very beginning, when you're just getting started.
