File New will create a new image in either RGB (color) or grayscale mode. You can't create an indexed image since GIMP will not know which colors your palette should have. Working in indexed mode is usually not a good choice. Instead work in RGB and convert to indexed just before saving the image.
How to create a new image:
The resolution is the number of pixels per unit. The default 72dpi (72 pixels/inch) is good if you want to create web graphics (you can alter the default value in the Preferences dialog). However if you are going to print your image, you will probably need a lot more than 72dpi. You have several choices about how you want to measure the resolution — pixels per inch, mm, points or picas (there is also an option which will bring up a dialog with even more choices). We suggest that you stick to ppi since that the most common format when you are dealing with images. You can have different X and Y resolutions by unlinking the chain, but that is not recommended.
You have two options: either you can set it directly in pixels in the top frame or in a real world unit in the middle frame. If you are working with web graphics we suggest that you deal with pixels directly. If you are going to print your image, then setting in a real world unit is the preferred.
Set the mode to either RGB or Grayscale.
Foreground takes the current foreground color from the toolbox. Background takes the current background color from the toolbox. White sets the fill color to white no matter what the toolbox colors are. Transparent prevents the image from filling. Instead you will get a totally "empty" image to start.
The Reset button resets all values to their default values. OK creates a new image.
Default Keyboard Shortcut: Ctrl-N