I totally agree that a good start is crucial for successful working with QGIS.
First of all, select your line layer in the layer tree:
Then you can either:
...and click on your desired line. It will turn red, and a window with the features attributes (ok, few in this example...) appears:
- with one of the Select features tools
you can select one or more features of the selected layer. These turn yellow in the map and in the status line you can see the number of selected features of the layer:
Then open the attribute table of the layer and at the bottom left select Show selected features, which should be self-explaining.
- The opposite way works too. Select one or more features in the attribute table and click on Zoom to selection, either in the QGIS main window or in the attribute table.
- Furthermore you can right-click a feature in the attribute table and select from one of the options Zoom to Feature, Pan to Feature and Flash Feature
I think you'll have guessed it already: This answer is not specific to linestrings, but works with points and polygons too!