I have a Layer and I have lines as if they were the sidewalks, so for every street I have two lines. The problem is that some of the lines are not joined, how could I join them?
Open "QGIS with Grass", open the "Toolbox" under the "Processing" menu. Under "Processing Toolbox" searc type "Clean". There is a tool from Grass that should help you.
More details about the tool are available here.