The approaches below are using QGIS 3.2.3-Bonn. Screenshots and other features may change or not be available in other versions.

Traditionally contours are labelled with the labels always upright in the uphill direction.

Depending on how your contours have been generated, you may need to use the Reverse line direction tool in the Processing Toolbox to regenerate the contours in the other direction. This can be done in place using the Edit Features in Place option at the top of the Processing Toolbox.

Under Layer → Properties…, go to the Label tab

Select Single Labels

Select Label with = ELEV (assuming that is the elevation attribute of your contour layer)

Under Buffer sub-tab, select Draw Text Buffer, which will create a thin highlight around each contour.

Under Placement sub-tab, select Parallel, and On line

Under Rendering sub-tab, select Show upside down labels = always, and Suppress labeling of features smaller than = 30mm (or as desired). The Show upside down labels is important if you want contour labelling to be consistently in one direction.