There are lots of visual design ideas to consider when making your icon. For inspiration and to see how other people approach making their icons, check out this range of tutorials on http://sixrevisions.com/graphics-design/50-excellent-icon-design-tutorials/. Don't worry about going through all 50. Pick and choose the ones that look interesting to you.
Some important ones I'll highlight are:
That may sound like gobbledygook. All you need to know is that if you draw an icon on paper and then scan it in to a computer using Photoshop or something similar, those are the settings that you'll need. If you're creating one from scratch using Photoshop, those are the settings that you need. RGB refers to Red/Green/Blue which is the system that computers use to show images on a screen (each single white pixel is made of 3 differing shades of red, green and blue pixels). It's called additive color, since we're adding the colors to make up an pixel. CMYK refers to printed images on paper, with Cyan/Magenta/Yellow/Black as inks that overlay each other. It works the opposite way, by subtracting colors in the visible spectrum using the inks and is known as subtractive colour.
(So why do we use RGB if we decide to draw our icon by hand on paper? Shouldn't it be CMYK? The answer is because we are showing the icon on a screen for the final product, so we need to make sure it looks great in the RGB color space, and not in the CMYK color space. Here's a great image showing the differences between RGB and CYMK. Color space is a fascinating and complex subject.)
Once you've got a 512x512 icon, then you can cheat by using the iconimator website.
Here's a bit more information about all the different size icons we normally need on the explanation blog post for the iconimator, but the website will automagically(tm) generate all the different icon sizes from a 512x512 icon. (-;