Assuming you already have Xcode installed:
Start Xcode, select “Preferences -> Locations”. Chances are that your Command Line Tools are not set. Select the suggested Xcode-tools location and you are done.
Additional:
You could check with:
xcode-select -p
which tools are set, mine showed:
/Library/Developer/CommandLineTools
After the location in the preferences was set, it showed:
/Applications/Xcode.app/Contents/Developer
(as it should)