I would recommend buying a book if you need to get to grips with Photoshop. I like the Visual Quickstart series, see here:
Link EditThere are a couple of ways of doing what you are trying to do. If you are trying to edit a drawing, then Photoshop may not be the best tool.
1. Open the image
2. Select the Lasso tool
3. Roughly draw around the picture making sure that everything you need to keep is within the line
4. Select Edit > Copy
5. Select File > New
6. Under "Contents" select "White", then click on "OK"
7. Select Edit > Paste
8. Now you can use the magnifying glass to zoom right in to view individual pixels. At this zoom you can use the eraser tool to erase everything around the edges of your image. This will leave you with just your picture on a white background. Make sure you reduce the brush size of your eraser to a size that is suitable - probably 1-2 pixels.