it is generally recommended to have a filter in place, more to protect the master lens from scratches, dust, the list goes on.
there are a few to choose from, vitacom, hoya, tokina to the more expensive b&w.
I recommend using the search function for this forum and check existing threads. Your question is very common. You may also read up about "ghosting" and "flare" - common issues arising around filters. Reading existing threads will give you plenty of information and opinions. For prices have a look at the Mass Sales or Classifieds.
When I step into the photography arena, friends told me get a uv filter to protect my lens. I saw also recommended B+W filters from Cathay Photo. Well apparently this filter is one of the best around in the market at the moment. Hoya, the cheaper but still relatively good comes in at 2nd on my opinion.
But regardless which you are using, it seems like under some direct light circumstance, they cause ghosts and flares, especially prominent in wide angle lens such as Tokina 11-16mm. It doesn't mean without filter will eliminate them, but I have tested against some flood lights on the side and without filter gives much better output then with a B+W filter.
In my opinion, you can put on a UV filter most of the time, but when u r find at certain shots flares or ghosts appears, take it off for awhile. Put on a hood to help prevent cracking your lens when you drop it.
This is my truthful and sincere opinion is, NEVER GET A CHEAP FILTER. If you cannot afford a filter, or not willing to get one, DON'T EVER SETTLE FOR SOMETHING CHEAPER. You are better off with a hood with no filter than a cheapo filter that gives u ghost + flares.