Click and drag while using thePaintbrushtool to create a line.
Likewise, click separately in the direction you want your path to follow when using thePenorCurvaturetools.
after you snag your line drawn, select it using theSelectiontool at the top left of the toolbox.
Then punch theSmoothtool, found by holding down theShapertool icon, if not already visible.
Click and drag over your line multiple times to smooth the path.
Whether for illustration or graphic design work, these tools will rid your work of uneven and shaky lines.
Enjoy smooth sailing in Adobe Illustrator from here on out, especially when it comes to line work.