Situation: We are making a flyer for an auto show. We need to isolate the car to the left for display on the cover of the flyer.
Your primary tool will be the Pen Tool (). If you are not familiar with the pen tool, try this tutorial. (It's for Photoshop, but the Pen Tool's identical)
Select the Pen Tool () from the toolbar. Click on the edge of the car. Carry on tracing the contour of the car. Don't worry about screwing up, you can refine your path afterwards. When you come full circle and are about to connect your last anchor point to your first, a white circle appears telling you that you are about to join the path and end it.
You can now refine your path using the Direct Selection Tool (). Select anchor points and move them snug up to the contour of the car.
If you have trouble selecting the anchor points; Select the original photo with the Selection Tool() and press Ctrl + 2( Object>Lock) on your keyboard. This will lock that in place. Since the car path is the only other object in the scene, it is the only thing that can be selected.
Once you have refined the path, press Ctrl + Alt + 2 on your keyboard to unlock the selection. ( Object>Unlock All)
Now that you've created and refined a path for the car, all you have to do is mask the photo to that path, here's how;
Select the photo and the path. From the main menu, choose Object>Mask>Make and voila! You have now isolated the car.
Now here's the part you don't want to forget;
Select the photo (yes it's still there) and the car's path again, and choose Object>Group to group the items together. Now you can move the car and the masked photo around as if it were one object.