Hoe can I use my iMac as a display for my MacBook

Please read Target Display Mode (Target Display Mode) to see if you can or not. My recommendation though is rather than buying expensive cables for a difficult setup and unreliable solution to simply buy an external display for the MB. Remember Macs are computers they are NOT displays for other devices.

There are only a few models of iMac that can do this. If your iMac is from mid 2014 there is a chance that it will work, but not with older models. Please check this link for reference and instructions

Use your iMac as a display with target display mode - Apple Support

To clarify, Mid 2014 iMacs were the LAST to be able to act as a monitor for another Mac, and could run no higher macOS than 10.13 High Sierra. Your Late 2014 was never supported with TDM, regardless of its macOS version.