If you have the Choice, USB or 3.5mm?
If you don’t have a microphone to start out with and so have a free choice in what you buy, then go for USB.
3.5mm microphones depend on the soundcard that’s built into your system, and in some cheaper builds they can be really poor. USB headsets by-pass that hardware, though, so you’ll get a more consistent result.
Can I use my Internal Laptop Microphone?
In general, the internal microphone that’s built into your laptop will be really low quality. I’ve never heard a PC-based internal microphone that was up to the task.
Within a mac, however, the internal mics are a lot better. Still not ideal, of course, but they can hold their own in a pinch. If it’s a mac microphone, or nothing, then go for the mac microphone. If it’s a PC microphone or nothing, however, I’d get a headset as soon as you can.
A Final Word on Getting Started
Having said everything above, don’t let ANYTHING stop you starting out in recording. The quality of your show is 90% content and 10% audio quality. That means that your eventual success is all about the practice you get in presentation skills and content creation.
You can upgrade a microphone within one quick purchase, but upgrading your presentation skills takes time and practice. So even if your mic is rubbish, start practicing now!