Honestly, these answers can easily be found in the user guide. Did you get to that?
To view your terrain at a larger size, and better quality, than your preview, you need to set your render settings, first, by clicking the Render Settings button on the Rendering Control dialog. You can set the size of the rendered image in the Render Settings dialog, plus other settings affecting image quality. You can probably get by with the default settings at first, apart from setting the size of the rendered image. Larger sized images will take longer to render.
Once you've set, and saved, your Render Settings, you need to set the Detail slider on the Rendering Control dialog. If you want to see the best quality, you need to move the Detail slider all the way to the right, which is the highest Detail settings. Setting the Detail slider to anything other than the far right setting will result in a less realistic image. This is generally only useful when you want to preview something quickly that doesn't involve a lot of detail (like the overall geography from far up), so whenever rendering a image to really view in detail, always set the Detail slider all the way to the right.
After those two steps, you can just hit the Render Image button on the Rendering Control dialog, and wait for the render to finish. This will get you started, everything else is in the documentation (please read, you won't regret, just take as slow as necessary) and what you learn through experimentation. Search the Terragen 9.x forums.
There are two files involved with saving a Terragen project. One file is the heightfield, the terrain file. You save that file by clicking the Save button on the Landscape dialog. All of the settings apart from the heightfield are saved by clicking the Save World item on the World File menu. To re-open your project, you have to open both files.
Good luck!
