well, you're off to a good start, and here are some pointers
attachments are easier recieved in jpg - cause they show directly in the browser. And look in tutorials for a river guide - but basicly
1. rivers flow by the easiest route from high to low towards the ocean
2. the only merge, they don't split
3. they don't go from ocean to ocean (as this would require perfectly flat land - canals excepted)