Obtain the proper information to make the calculation. Whether you are using data from homework or from experiment, use the thermal resistance equation. Thermal resistance equals temperature difference along a heat flow path divided by the power dissipated along that path in degree Celsius per watt (°C/W). You, therefore, need to know temperature of heat sink (Ts), temperature of ambient (Ta), air within enclosed natural convection environment, and power dissipated (P) in watts along the path of heat flow.
Convert the quantities to correct units. The unit of thermal resistance is degree Celsius per watt (°C/W). You will need to have "Ta" in degree Celsius (°C), "Ts" in degree Celsius (°C) and "P" in watts (W).
Perform the calculation. Subtract the value for "Ta" in degree Celsius (°C) from the value for "Ts" to obtain a single value for "delta T" in degree Celsius (°C). Divide the value for "delta T" in degree Celsius (°C) by the value for "P" in watt (W) to obtain a single value for heat sink thermal resistance in degree Celsius per watt (°C/W).