That will depend on the type of format you want to save the file in.
If you want to use JPG then Gimp provides a dialog box just for that. To use it you have to save the file as [Save As] (dont save over the same file). Confirm exporting if the dialog pops up. Then in the next dialog box make sure you have enabled preview, and now you can change the quality of the image to reduce the Kb.
NOTE: If you are using Gimp2.2x you will need to click [Advance options] in the above dialog box to open up options for changing the quality/Kb. Gimp2.0 used to show directly all the available options while Gimp2.2x hides it.