well my english is not this well and i dont like to explain it once more (i have to explain it 4days a week)
so check out
these tutorials
PS:
To create transparency in a new file in Photoshop:
1. Choose File > New.
2. Enter the desired width and height.
3. Select Transparent for the Background and RGB for the Mode, and then click OK.
4. Create the image, leaving the checkerboard visible in all areas that you want to remain transparent.
To create transparency in an existing file in Photoshop:
1. Open the file in Photoshop.
2. In the Layers palette, double-click the italicized layer, which is usually named "Background." If the file doesn't include any italicized layers, it already has a transparent background: Skip to step 4.
3. In the New Layer dialog box, rename the layer, and then click OK. The file's background is now transparent.
4. Using the selection tools, select and delete areas of the file to create transparency. The checkerboard indicates transparent areas of the image.
5. Choose Image > Mode > RGB, and then save the file.