In internet, you may find a wide variety of forums and howtos (also here!) on photographic edition, either to increase the dynamic range to produce an HDR image, or to convert it to black and white, or even to tint it to sepia, for example.
But one of the most common (an needed!) actions is usually bypassed: resizing. I indicated in some previous post that image size is a complex topic. However, I will present the basic process to (for example) reduce image sizes to upload them to your favourite profile (facebook...) or to send it by email without blocking your output tray...
Process in GIMP
Open your image. In the information bar where the file name is written, you can also see information on the size (in pixels, single colour dosts) and colour space.
Open the command "Image - Resize Image", you will see a tool window where you can set new dimensions and resolution, if you want to change it. Let'sstart with this latter.
Digital images are produced as a grid of colour points. Resolution value indicates how many points fit in an inch (or centimeter) of the image.
As practical values, choose 72 or 96 for screens, between 150 and 200 for large-format printing (posters) and 300 - 600 for standard photo printing.
Now, you may introduce the target size in your desired units - be it inches or meters. When you click on "Resize" you will already get your new file...
If you are working for a website, you may skip all this and set the dimensions directly; choose values around 800 - 1,200 width - this will fit most modern monitors.
GIMP provides at least three flavours of interpolation: "None" will just make basic calculations, and only information from some dots will be maintained. "Linear" makes some averaging between remaining dots and their surrounding ones, so that this is a good option to reduce images and keep some texture.
Last option is cubic interpolation, which uses more points around to provide a finer output - even for enlarging, too, if this is your case.
Anyway, remember that when enlarging, GIMP (and any other edition program!) must make up all "new" added points, based on existing information. You will not get more information, or better resolution or definition... but a bigger file size.
And remember to save the image with a new name! so you will not lose the original image...