A polarizer would be a good start. Not only can it reduce glare and enhance colours, it reduces light entering the camera by about two stops. That way it can also be considered a ND filter as well

Do not go too cheap, get a decent brand.
From there...it depends. ND filters come in two main types: screw in and square. The screw in is as it sounds, on the end like any other filter. The square ones need a holder. You can look at Cokin, Kood or Formatt/Hi-Tech for fairly inexpensive kits of filters and holders.