Fish farming might be a good idea for a small farm holder or interested entrepreneur. The type of fish to raise depends on the topography (mountains, hilly, or plain area); climate and temperature; and type of water available (still water like pond, or running water).
If you are starting to think about fish farming, first step is to fix a location, then choose proper type of fish based on characteristics of that location.
For fresh cold water with continuous flow, trout farming is popular.
There are other options for still water, different topography, etc. You can also visit some fish farms to get more ideas.
Here is a resource for simple fish farming process:
Nepal Aquaculture Promotion Group (NAP-Group)
http://www.facebook.com/groups/223016191061632/ could also be good resource.
Another strategy can be: Research about market demand and prices of different types of fish currently available (or not available) in the market. Then decide on the most profitable type of fish. With your research, you may even discover that importing a high demand product from abroad might actually be the most profitable option! Your good research will provide you the best answer.