This subject tends to be marginally more controversial than Brexit. Some people use them all the time, some only rarely, others never. If you use one, choose a high quality filter that is both multicoated and has an additional layer that makes it easy to clean. Examples include the MRC and MRC Nano ranges from B+W, and the HD and Fusion/EVO ranges from Hoya. Hoya Pro-1 and HMC filters are multicoated, but lack the easy-clean layer. Both companies also make uncoated or single-coated filters that you probably don't want - multicoating has a big impact on reducing flare when shooting into a bright light source. Use a hood for shading your lens from out of frame light sources and for impact protection in certain circumstances. A filter is more useful for protecting against wind-blown grit and rain, etc., and might save your lens from careless cleaning in the field. If you do choose to use one routinely, be aware of situations where it is more likely to have a negative impact and consider removing it (cityscapes at night with many bright light sources, shooting with the sun in the frame, etc.).