What power cable to I need for my new internal hard drive?

That is a SATA hard drive and requires a SATA data cable and a SATA power cable. The long slot is the power connecter and the short slot is for the data cable. If you power supply does not have a SATA power connecter, all you need is a 4 pin "molex" to SATA power adapter. They are available at any computer supply outlet. Staples, or Office Depot, or any good computer repair shop should have one. The little port that you have circled in the photo does not need a connection.