After effects is asking for cuda. But cuda is for nvidia. But i have the ATI Radeon then what should i do ? please help me.
After Effects is not asking for CUDA. It is merely saying that if you want to accelerate the (now-obsolete) ray-traced renderer, you'll need an approved GPU and CUDA. You should definitely Avoid installing NVIDIA CUDA drivers on computers with AMD GPUs!
The ray-traced renderer is, as I said, considered obsolete now: http://motionworks.net/after-effects-3d-future/
And AE uses the GPU for little else: GPU (CUDA, OpenGL) features in After Effects