Depending on the target you are interested in imaging and the sensor you are using, you may be able to use a variety of filters.
If you are interested in emission nebula, then usually a contrast-enhancing filter helps. These come in two strengths usually, a lower strength that lets more of the continuous light through (which helps keep star colors intact) and a high strength version which helps eliminate more of the background skyglow and reject more of the sodium and mercury vapor city lights.
If you are looking at a nebula that features light primarily in Hydrogen Alpha or Beta or Oxygen 3, then you can use a notch filter just for that specific wavelength. These are the best option for eliminating nearly all the skyglow and light pollution in an area. The narrower the bandwidth is better for these cases. Popular bandwidths are 7nm to 5nm.
There is a third option which is a very gentle filter based on didymium doped glass which can help reject some yellowish skyglow. It's a commonly used filter to enhance the red colors of fall foliage.
Choosing the right filter depends on your imaging sensor, too. If you are using a one-shot color sensor (DSLR and film are examples of this) then the former contrast enhancing filter would be more appropriate than a notch filter.
Note that if you are going after bright targets like the moon and planets, filters are usually not a big issue except to minimize telescope artifacts like CA and purple fringes. Then you would be considering IR and UV blocking filters and maybe minus violet filters.