So you're trying to add a watermark with a text/logo to your photo and print it on A4 size, correct? If so, keep reading
There are many ways of adding a watermark into an image. The easiest will be a simple text without a logo etc.
For that, just open up the image (I am assuming you already have it as a file on your PC, if not you'll have to scan it first) you want to have a watermark on in Gimp, then
-Click on the text tool
-Click on the image and add your text by typing it into the small pop-up box. You can also move the text around with drag&drop.
-Take a look at your layer's dialog (), click on the Text Layer and lower the "Opacity" slider down to whatever percentage you think looks best (should be around 40% I guess)
If you want to add an image/logo to your image as well, just open up your image (the one you want to put the watermark on) in Gimp and then
-Click on "File > Open as Layer"
-Select the image/logo you want to use as a watermark and hit "Open"
-Now, do the same thing with the opacity slider again (see above)
-Optionally, use the move tool () to position your logo correctly.
If you are done, go to "File > Save As" and save it as "Image_Watermarked.png" or similar. It is important though that you do NOT overwrite your original file because you can hardly remove the watermark afterwards.
You can now easily print this picture or upload it to a website etc.
If this was not what you wanted, please give us a more detailed description of your problem.