The most important thing to consider when launching a podcast, is to consider how passionate you are about the subject matter of your podcast.
If you're not passionate about your topic, your interest in podcasting about it will quickly fade.
Having said that, this is how to launch a podcast in 5 steps.

1. Get a microphone.
   I'm not talking a $400 studio microphone, but at least something with decent sound capabilities.
   Do not, under any circumstances, rely on your computer's built in mic.
   Yes, people will stick with a podcast that sounds a bit weak, if the content is spectacular, but not for long.
   I suggest the Audio-Technica 2020 USB mic (approx.
   $100 as of this writing) as a good place to start from.
   There are plenty of less expensive mics out there, just don't use the one in your computer nor one of those long skinny beige ones that you get for $5 from the bin at the computer store.
   
2. Decide on a schedule.
   Weekly, monthly, daily or whenever you feel like it.
   I suggest weekly since most people are familiar with that schedule.
   Whatever you do, make sure that you stick to your schedule.
   
3. Record your podcast.
   You can start off with a free application like Audacity, but for a few bucks you can use something more professional like Adobe Audition or Garage Band.
   
4. Decide on a distribution network.
   My suggestion, is to use a dedicated podcast hosting provider.
   Sure, it costs a little extra, but a lot of regular webhosting providers do not support hosting your podcast files.
   Use a dedicated or specialized hosting provider like libsyn for hosting your podcast files, and the iTunes music store for distribution to a wide audience.
   iTunes, is the easiest place to host your podcast and is usually the easiest for your listeners to use.
   Whatever podcast hosting you choose, will help you with uploading the files.
   
5. Promote your podcast.
   Figure out who needs to listen to your show, and put it in front of them.
   If you've chosen a distribution network wisely, you've also made it easy for people to subscribe to your podcast show, which will make it easy for your audience to grow.
   

Some may argue, that promoting the podcast to a sizable audience, is more important that deciding on the content of the show.
There is some validity to this.
Some audiences are too small to make them commercially viable.
However, podcasting should be about your passion and not about the size of the audience.
It's up to you to decide whether you want to work on something solely based on the size of the audience that would listen to it, instead of something that you are passionate about.
If you're not moved by the subject matter that you're talking about, how can you expect your listeners to be engaged? How do you expect to continue recording shows that just do not interest you?