Turn on computer, plug in power to your portable hard drive, plug in USB (or eSATA) for your portable hard drive - wait for windows to load driver (new hardware found) - discover hard drive in "My Computer". Drag and drop the files you want - or use save to, or...
First, if I understand correctly, you bought separately an HDD and an enclosure. So first you need to follow the instructions with the enclosure on how to mount your HDD inside, and how to connect it to your machine. It will be detected and a driver loaded if necessary. However, you will NOT find it sitting in My Computer ready to use. Any new empty HDD needs to be Initialized (that is, Create a Partition and Format it) before you can use it.
See WyomingKnott's sticky at the top of the Storage ... General Discussion forum here:
Put the hard drive into the enclosure and connect it to the Pc. Make sure that the drive is initialized , partitioned and formatted and that a drive letter is assigned to it. Once the drive is connected you can do all that in Disk Management.
Right click on computer in the start menu and select manage then in the next window select Disk Management, when in Disk Management you just have to follow the steps as the initialize window shopuld be open when you get there.