Ok, so there are two main types of filter system that will fit onto the front of your lens.
Screw in type: These are circular filters and screw directly onto the screw thread of your lens. These are great if you only need one type of filter at any one time. For example, you might want to use just a circular polariser, or just a Solid Neutral Density (ND) filter. However, as soon as you wish to use graduated ND filters, this system falls down.
Slot in filters: This is by far the most versatile filter system available. In short, an adapter ring of the same filter thread size is screwed onto the front of the lens, and to this the filter holder is attached. The filter holder usually has room for 3 square or rectangular filters, which slide onto the holder from the top or bottom. Then the type of filter you use at any given time is totally up to you, with the combinations being also at your discretion.
Types of filter: A lot of people like to use UV filters, and indeed these tend to be the circular screw in type. Ultimately these provide zero noticable change in image quality, having only an adverse effect as sensors have UV filters on them already anyway. I suggest using these only for protection in harsh environments.
ND filters are great as they (Ideally, but rarely ever seen) don't affect the colour balance while extending the exposure by the prescribed amount of stops. A big stopper being ~10 stops is best used during the day to evening, as during the night, you will be there for literally hours. During dusk and after sunset, a 3 stop ND filter, or the combination of a few to give more than 3 stops can be handy
Circular polarisers are good for use during the day. These do a number of things, without me getting into the physics of how they work
They act to reduce reflections from reflective surfaces, boost saturation of certain colours, and enhance contrast between sky and cloud. These are arguably best used when at 90 degrees to the sun, but it's totally up to you. Personally, I rarely use one, and I take predominantly landscapes.
ND Grad filters. These are, in my opinion, a type of filter that anyone interested in landscapes should have. These are rectangular filters with a ND area at the top, and a transparent area at the bottom, with the aim being to equilibrate the contrast between a bright sky and a dark foreground. These are most useful and I would highly recommend getting some if you intend to shoot landscapes.
As such, and indeed for sticking within your budget, I would suggest looking at the Cokin P system. This will consist of a 77mm filter ring, Cokin P filter holder and what ever filters you require. It should be noted that for the 10-20mm, it would be handy to get the wide angle filter holder, as this will avoid the edge of the holder encroaching the frame. This however will limit you to using 1 filter at a time. Therefore if you think you will need more than one filter at a time, look at the hitech filter system instead.
Hope that helps
