What cable do I need to connect a new mac-mini to a 2011 macbook pro ?

See https://support.apple.com/guide/mac-help/transfer-files-mac-computers-target-disk-mode-mchlp1443/mac

Since your Mac mini (you said "new," I'm assuming latest model) uses Thunderbolt 3 which has a type C port, you need this adapter: https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter

As well as a regular Thunderbolt cable: https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m

One option if you are open to it, remove the drive from the

MacBook Pro and use a SATA-to-USB3 adapter cable (usually

$10-15 on Amazon) and then use Migration Assistant. The drives

are pretty easy to remove from 2011 MacBook Pros and have swapped

drives multiple times on mine.

https://eshop.macsales.com/installvideos/macbookpro_17_unibody_early11_hd/

Do you have a current Time Machine backup of your 2011 MBP? If so, then just plug the Time Machine backup into the new Mac Mini and migrate from it.