This is very simple to do and I only show the basics, Once you can do this you can branch out and add more detail to the text or even add some extra features to it.
Open up GIMP and hit File, then new.
You can set this to any size but I am just going to do it 500 by 500.
Further on you will notice there is alot of space at the bottom. I wont be using that extra space but you can to add some more detail if you wanted to. If you just wanted the text by itself, You could set it to something smaller like 500 by 200 or you can just have it 500 by 500 and resize any extra space out when your done.
The default is generally a White layer, We do not need that so press Ctrl + L to open up layer menu. Click on the layer (It will be the only one there) and then in the bottom right corner of that box, Hit the "Delete this layer" button. When you do this, There will be squares. Thats basically a transparent images, There is nothing on it and if you were to upload that, It wouldnt come up.
Now click the "New Layer"button in the bottom left. Make sure its set to transparency.
Hit the "Text tool" Its the icon with a "A"
Click and drag on your image to select the area you want the text to go.
You can make this bigger later if you wanted to. Type what you want in that spot and then you can later use the move tool to move it around. On the image to the right of this, I marked a red square. That area is used for changing the text, You can change the font, the size and even how far apart the letters are away from eachother. You will also see a color button, When you first create a text box, The color will be the the color you have selected at the top, but if you wanted to change the color, You use the color button at the bottom.
When you are done changing your text and moving it to where you want, In the window where your image is, Go to "File" and hit save as. give it a name and you can even select a location, but when your naming it, Whatever you call it you need to put .png at the end. So if your image is about "Puppies" You'd need to name it "Puppies.png"
This allows GIMP to save it in such a way that it wont fill the background with white and mess up the transparent background. There is other save types like .tga but I find png would be the best.
Thats pretty much there is too it. If you have any questions feel free to post a comment below and I'll answer them.