also be aware, grads and polarizers only work properly with lenses whos front elements dont rotate when focusing. imagine for example you have a graduated ND filter and want to give the sky some depth of colur. If you have a lens where the front element rotates as you focus that filter is going to turn and your grad can end up at the side or the bottom ( or some other position other than at the top ) and its pretty much impossible to adjust it once youve focused.
same with a polarizing filter which is made up of to pieces of glass and one of which rotates around to give the polarized effect . as soon as you focus its going to lose the position you have set the filter in if your front element rotates. ( this is all based on screw in filters ).
your two kit lenses are most likely around 58mm filter size ( you would need to check ) but being kit lens theres a good chance they have a rotating front element when focusing so unless you plan to upgrade lenses then you are going to have problems.
actually just double checking even square filter systems like Cokin screw into the front of the lens so your going to have the same rotation issues.
ND filters are not rotation reliant and can work any way round ( the non grad ones ) so they are fine.
also UV filters ( pretty much redundant to use on most modern cameras now due to lenses coatings but very cheap and worth having just to protect the front element of your lens from dirt and dust ( easier to replace a £10 UV filter than a £200 lens ).
There are a whole other myriad of coloured filters, Starburst filters , warm up filters and more but for 99% of the time most people stick to NDs, polarizers and UV's
taken from Cokin website
There are over 140 filters to choose from in the range.
The most popular are:
- Polarizers - Designed to eliminate unwanted reflections and darken skies.
- Graduated - Add colour to a scene by using a Cokin graduated filter to enhance a dull sky, or use a Sunset filter to add warmth.
- Pastels and Diffusers - Ideal for portraits, still life and landscapes, to create a moody atmosphere and a softer effect.
- Warm-up Filters - Add warmth to a portrait shot, giving improved skin tones.
- Colour Filters - Colour correction filters are available, as are Neutral Density filters and filters specifically designed for use with black and white film.
- Optical effects - There is a wide range of filters available to add special effects to photographs, such as Starbursts, Multi-image, close-up, Rainbow, Double exposure etc
this page pretty much covers all the filters available
http://www.cokin.co.uk/pages/filters.htm
like i say though if you have rotating focus front element lenses it wll limit use of certain filters