StuffIt Expander will change files ending in ".SIT" or ".BIN" or ".HQX" or ".SEA.BIN" into files you can use on your Macintosh. ("SEA" by itself doesn't require StuffIt.)

DiskCopy will allow you to make disks from files ending in "IMG".

Once you are able to open DiskCopy and have some disk images ending in "IMG", fiddle around with the menus in DiskCopy and you'll figure it out in no time.