Note: This is my style of vectoring; real vectoring is actually much cooler looking but harder to do. Alright, you'll have to know how to maneuver your way around your Photoshop (and yes I will be using Photoshop) for this tutorial as it's already hard explaining what to do without having to walk you through basic functions.
First thing's first, your image. I like using simple images of people in fun poses; the end result is always more exciting that way. For this tutorial I'll be using this image of Hannah Beth. For other cool photos like this one check out Be-Scene.org.
I re-sized the photo a little just so she isn't too large; Height at 450 pixels; and now we can start.
Create a new layer to work with Layer > New > Layer... > OK and then grab your Polygonal Lasso Tool (L).
Now, you're pretty much going to be tracing and then coloring in each part of your image. I always start with the skin; it's a good base layer and since you're going to have multiple layers and the skin is usually under clothes and hair, it's a good place to start. So, starting with her face, and with your Poly Tool, tracing along the exposed part of her face. Doesn't have to be perfect just as long as you've got some shape in there.
Next grab your Eyedropper Tool (I) and click on her face to get the color of her skin. I usually do this but you can of course use any color you want for the skin. Now with my Paint Bucket Tool (G) I fill in the selected area. I do this to all the areas where her skin is showing; sometimes I create new layers for the limbs, depends on how complicated the clothes and poses are. You should have something like this when you're done with the skin:
Now we move onto the clothes. Create a new layer on top of the skin layer Layer > New > Layer... > OK and grab your Poly Tool again. This time trace out her dress/shirt. Now, sometimes you won't be able to see all of the clothes because of hair or objects, this is fine as it'll be covered in the end anyways. As long as you have a basic shape of the part you're good.
Again with your yedropper Tool get the color of the clothing and then fill it in with your Paint Bucket Tool. You can go crazy with the colors of course though; sometimes I fill the area with a pattern.
With the hair you might have to hide the shirt layer for a moment so you can see what you're tracing. Create a new layer on top of your skin and clothing layers and then get to work with your Poly Tool again.
Color the hair any color you want. You can get a little more detailed and add the design on the shirt or the glasses, you get the idea. A finishing touch I like to add to my vectors to give them a sort of line art touch is to go into each individual layer and add a black 1px Stroke or if you want a less severe look make the stroke a shade or two darker than the color you used to fill in that layer. Here's how mine turned out;
