Like most of Southeast Asia, Singapore is generally hot and humid. Average temperatures hover around 86 F/30 C and seldom dip below 75 F/22 C. November and December is the rainy season.
June-August is considered to be the best time to visit, but even then it rains often. Don't let the climate stop you from going, however. Most buildings are air conditioned (to the point that you may want to take a sweater), and pains have been taken to make everything as comfortable as can be, all things considered.
When it does rain, it's generally only for a short period.