|The answer depends on what files you want to transfer.|
Once the external drive has been partitioned & formatted you can simply move the contents of your documents & pictures & music & Movies etc.
Installed programs are a different matter. Those have to be uninstalled from C and re-installed to the new drive location.
With a 98% full drive to begin with you may very well need to do some tidying up first.
See if you can run Disk cleanup first to free up some space.
Goin' Fishin' (Some day)