After you read the river tutorial, follow these simple steps and you should have decent enough rivers:
- Pick the places where it rain the most (Is the mapped area big enough so that you have a significant rainfall gradient?)
- See where your basins would be located (read the tutorial)
- Make your rivers seek the ocean (or in some rarer cases lake) following the terrain (never climbing)
- Do not split your rivers. It may be tempting, but stay strong.
- Nearby rivers tend to merge.
- Consider deltas.