Hello indranil80.
Welcome to JJMPF. Pleased to meet you. You will find our fellow JJMPF Members here very interested in your progress in the hobby of photography.
One thought I have is that, within limits, you may have the capability you seek already. I will assume that your Canon came with a kit lens. With a really
sturdy tripod and shooting somewhere around f8, you should be able to get very good shots of static images into the night. You will need a remote release.
For very little money you can also buy a fast 50mm lens. With this optic you can shoot right on the edge of darkness even without a tripod, given good shooting technique. Budget allowing, a fast 35mm lens would be very handy in the same sense, and would provide a more "normal" angle of view for your Canon.
Then there are some more costly, "really-right" solutions. Our fellow Members can elaborate better on those than I can.
Again, welcome aboard, Indranil.

Dear friends
I am relatively new to the world of photography and had bought a Canon 500D about 4-5 months back. I am interested in low light photography (mainly cityscapes and landscapes -sunset,sunrise).
I request your suggestions for lenses that are best suited for same. Of course - best lens at minimum price 
Thanks in advance