I’d start by saving them in a lossless format as @fubidou mentions. You could go for .wav file but I’d suggest .flac it’s a lossless format, is open source and ends up being slightly smaller in filesize than .wav too.
First, keep these original unrestored copies no matter what. You can’t “undo” bad noise reduction and restoration once it’s saved out to the file.
I’d start by using Capture Noise Print in Adobe Audition on just the silent part of a tape. Then run the Noise Reduction tool on the recording and tweak settings from there. I really wouldn’t go too crazy with effects to improve cassette tape audio as most of the audio will be acceptable.
If you have any sample recordings feel free to post them here for even more detailed advice