Turning an old internal secondary hard dive into a external one.

Can somebody help me with how I turn an old internal secondary hard dive into a external one. its currently in the old tower.

 

Assuming it is a SATA hard-drive, they make "cases" for them that allow them to plug in via usb.
Something like: USB 2.0 to IDE SATA External Hard Drive Enclosure

 

You need a hard disk enclosure - if it's an old PC you may find it's not using SATA connections but the older IDE connector
SATA vs IDE Explained | IDE vs SATA
shows the difference (the IDE connector is around 4cm wide and the SATA is about 1cm wide - both types will have a second cable connected which is power).

For the older style you'll need a "IDE Hard Drive Enclosure" which then connects to your PC via USB (and as vyvance says these also exist for SATA) - however the IDE versions are more expensive as they are getting rarer (but you can probably find cheaper ones on Ebay).

 

Should be SATA as it is only 3 years old