It sounds like you're trying to make a collage of terrains to form a heightmap and then use that heightmap to create a shaded image. Coloring can be applied to the image by manually painting a color layer or by an additional collage of real-world imagery.
For any collage project, you'll need things to collage together, tools to do the collage, and an idea of what you want the result to look like (that last one is the hard part).
First (and by far the most important), start with a sketch that shows the basic shapes of the landforms for what you're trying to achieve (ocean, plains, mountains, etc.) This sketch is what you'll be working toward. Try to draw in any major rivers if you can because those will help to line up the terrains you pick. I say do this step first because you can do the collage with any number of materials, tools, and techniques. Here, you're interested in a digital image, so that decision will necessarily limit your choice of materials and tools.
Tooling is the second most important part of this activity because the tools will suggest what you can do. It's way easier to use Photoshop (or The GIMP or Krita or any image editor that supports multiple layers and at least 16-bit color depth) than it is to try to use Microsoft paint.
For the heightmap part, go to your favorite terrain browser (Turambar mentions several) and find areas that look roughly like what you want for individual parts. Sharp and young mountains might be something like the Alps, Sierra Nevada in California, or the Andes in South America; Old mountains might look like the Appalachians or the Urals; plains might be the Russian steppes, Southern Africa, the Pampas in South America, or the Nullarbor in Australia; forests might include any amount of green areas (lots of terrains are covered in forests). I recommend getting everything at the same zoom scale to ensure consistency of features.
When you find an area that will work for part of your map, export that area from your terrain browser as an image. Import that image into your image editing tool as a new layer. You can rotate imports to get things to line up with your sketch. Setting layer opacity to something like 60% while you are aligning things will make it easier to see where things are going (set it back to 100% after it's in place). Your goal is to roughly cover your map with overlapping images.
After the images are in place, use a soft eraser around the edges of each image to erase the hard edges and get the layers to blend together. If necessary, go find some terrain elements that will merge the idea of two chunks on your map (for example, a hard mountain range and a flat plain might benefit from some transitional hills; similarly, pasting some meandering river terrain might work for the lower reaches of a river valley). There are tools that can enforce river flow on a heightmap (Naima mentions Wilbur as an example) and may increase your plausibility of your terrain at some significant cost in processing time/
At this point, you should have a stack of grayscale images that work as a heightmap (or bump map as it's commonly called in graphics circles - yes, I know that a bump map only affects surface geometry while a heightmap is surface geometry, but close enough). I recommend exporting it as a single image and looking at it in 3D with lighting in your favorite 3D tools. Save the lighted flat grayscale image to a new file.
In your image editing tool, take the lighted image that you formed above and import it as a layer; set the blending mode to multiply. On a layer below that, paint the colors that you want on your map. On layers above that, add labels and borders and all of the frills that turn your picture into a map.
If I was unclear about any of the above (and I likely was), please feel free to ask questions.
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