First, try shooting some stuff with just the kit lens, it's versatile enough for some great experiments.
For low-light conditions; the wider the aperture on the lens (the lower the f.number) the more light can be let in, and therefore the more versatile it is for low-light. Also, the wider the aperture, the shallower the depth of field... generating that film look you're after. This rule also works the opposite way:
[smaller aperture (bigger f number) = less light, deeper focus]
The f.number on the lenses you can buy refers to the widest aperture it can be set to. If it's a zoom lens, you might have 2 f numbers, these refer to the lowest f number that can be set the the two extremes of the len's zoom.
As a first additional purchase, I'd recommend the 50mm f1.8 (or the f1.4 if you've got a bit of extra cash)
Happy shooting,
Smith