There were good tips and suggestions in the comments as well, especially this technique to adjust the levels using the Curves dialogue, shared by justacat. I'd mark it as intermediate, not beginner -- definitely worth checking out.
If you're wanting more resources and tutorials, have a look at the aforementioned icon_tutorial. Lots of good stuff in the memories.
If you want to practice your icon making and get inspiration from other poeple, join multi_pass! Each week, a photo is posted of some character or other (last week, Leeloo, this week, Riddick) for us to make an icon of. Post the result and there's a very low-key competition at the end of the week. I don't really care about the contest, though. The best part is seeing what other people come up with. Often, they'll post a short "how-to" explaining what they did.
Do you have any cool tips or favourite techniques for icon making? I would love it if you would share them, either here, or in your LJ. (But please post a link here so I can find them all. *g*)
Here's one from me:
You could just take a colour image and change the mode to greyscale, but you'll get a much better image if you follow these simple steps. This is for Photoshop -- not sure if this will work in PSP.
1. Select Image -> Mode -> Lab Color
2. On the Layers palette, click on Channels
3. Click on the Lightness layer; the other layers should turn off.
4. Copy that layer, paste into a new document, and Bob's your uncle.
Here's Odysseus in color, simply switched to greyscale mode, then done using the Lab Color method. (The difference isn't all that noticable at icon size, but try it on a larger pic and you'll see.)