how can i open my office documents on my new iMac

Microsoft Office documents can be read and written using Microsoft Office, the free LibreOffice and OpenOffice packages, and many can also be read and written using the (free with macOS and iOS) Apple iWork Pages, Numbers, and Keynote apps.

If you specifically want the Microsoft Office apps, you will need to acquire and install Office.

Whether you can transfer a license for Office depends on various factors, on the version of Office on the previous system, and that question as well as other support questions are best directed to Microsoft Support.

With macOS Catalina, you will need Office 2016 (no longer sold, still supported), Office 2019 (still sold, still supported), or a subscription to Office 365.

Old Office versions prior to 2016 are no longer supported, and variously don’t work with older macOS versions, and won’t work on macOS Catalina.

In short, check with Microsoft.

If you are frequently exchanging MS documents in their native document format with other Microsoft Office users, then you should be using current versions of MS Office for Mac (e.g. 365, or 2019), or the free LibreOffice which produces the best conforming MS document formats without translation. LibreOffice applications are purposefully clones of MS applications.

Apple's Pages, Numbers, and Keynote open, and without changing the original MS document, translate it into Apple internal document formats. Apple's internal document formats must be translated again to export back to MS Document formats. There is no guarantee that the translation process will be accurate in either direction, or that the exported documents will even resemble what they looked like in Apple's applications, when reopened in MS applications. Apple's applications are not MS clones.

Look into the Free LibreOffice, it handles MS Office files…

https://www.libreoffice.org/