how i connect new macbook pro 2018 to a imac 2013 display ?

Hi,

First of all, as Target Display mode document says, if your iMac is Latina Display, you can't use it as external monitor of MacBook Pro.

You need to get USB-C to Thunderbolt 2 adapter and Thunderbolt to Thunderbolt cable.

https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter?fnode=8b

https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m?fnode=8b

For more info., read Target Display Mode document.

Use your iMac as a Display.

https://support.apple.com/guide/mac-help/use-an-imac-as-a-display-mh30822/mac