No problem.
The border around your resized layer is just a layer boundary. It isn't anything visible if you decide to export the image to .jpg or .png format.
You can disable the visible boundaries from preferences:
![Image](http://gimpchat.com/files/2783_Preferences-Appearance.jpg)
When you are happy with your edited image you save it first to native format.
File->save as: yourimagename.xcf
This ensures that you can continue manipulating it later if needed.
After that you can use File->export as: yourimagename.jpg for example.
You can pick the image format from the drop down list, but it's faster just to type it after the image name.
A pop-up window appears. I recommend to raise the quality somewhere between 92-96 with .jpg, click "save defaults" and then export.
If your image has transparency, export to .png format. I'm using compression level 2 normally for .png
Read about most common image formats when you have time, to learn the difference and restrictions.