You may leave the sukka due to extreme cold or heat, rain, or bees. You should not stay in a sukka if it is raining or very cold or will otherwise make people suffer.
If it is raining on the first night of Sukkot and the rain is sporadic, wait until midnight before saying kiddush indoors.
If it is raining on the first night of Sukkot and the forecast predicts rain all evening, you do not need to wait to see if the rain will stop before saying kiddush. Instead:

• Say kiddush in the sukka (without saying leisheiv ba'sukka), even in the rain.
• Wash hands and say ha'motzi.
• Eat at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread in the sukka.
• Go inside the house and finish your meal inside.