samsung tablet GT- P3113 have OTG USB support? Does the samsung tablet model # GT-P3113 have OTG USB support already built in or do I need to get the cable?

Am thinking of getting a keyboard for my tablet.
According to the USB Host Diagnostics site, YES.
Check the link for the ROMs (Build IDs) that supports OTG.

If your phone has root, you can also check out USB OTG Helper [root].