See my answer here for a description on how to calculate land surface temperature, using the simplest approach.
However, the Landsat 8 TIRS sensor is currently not fully functional, and it has been in that state since 1st of November 2015. The current plan is that the data acquired in the 4th quarter of 2015 is to be re-released with the issue fixed during February 2016, and the data acquired in first quarter of 2016 will be released in April 2016.
The information above comes from the Landsat mission site where the following text can be found:
Landsat 8 Thermal Infrared Sensor (TIRS) data continue to be collected with the scene select mirror encoder electronics disabled (mode 0). While in this mode, the TIRS line of sight model (LOS) will be regularly updated and modifications are being made to automate revisions to the LOS in the Level-1 Product Generation System (LPGS).
Landsat 8 Operational Land Imagery (OLI) and TIRS data that have been collected through the 4th quarter of 2015 (October-December) will be reprocessed into nominal Level-1 products containing valid TIRS data, and will be available in February 2016.
TIRS data acquired during the 1st quarter of 2016 (January-March) will be reprocessed and made available in April. A strategy is being developed for generating near-real time products moving forward while operating in mode 0. More details will be posted on the Landsat Missions Web site as they become available.