The negatives that are a bit thick(dark) is a sign of overdevelopment. You can decrease the developing time or temp. It could also be that you over exposed in camera, but the film edges look a bit darker so I assume it's probably over development. Fog is also a possibility with expired or improperly stored film.
The lines may be scratches in the emulsion as they are black. Dust and dirt would be white when inverted on the computer or wet printed. You would want to check with a loupe. If you don't have one a camera lens filpped works fine, or even a simple magnifying glass.