Going hometown soon so I need to know what ND filter do I choose?
I will go get Cokin structure, but what are the nice filters to get if my objective is
shoot summer day sky and landscapes? Maybe sun rise and sunset also.
I think you should use circular polarizer filter instead of ND filter.
ND filter is designed to reduce the amount of light passing throught the lens so that a longer exposure time is required. So it may give you unwanted under-explosure with same shutterspeed/aparture unless you purposely want to use slower shutter to create blur effect on moving water or etc.
or you can use ND graduates to balance / tune down the sky. sunset graduate may help in sunset shoots as well.
ND filters are meant to reduce the whole pic's amount of light entering, similar to us wearing sunglasses. ND filters can also create the smooth silk-like effects of a flowing stream or waterfall.
U should get a circular polariser instead as these will enhance colours of objects as you wish, such as bluer skies, greener grass, etc. These polarisers will also help u take pics through water and glass to reduce reflections.
The usage of circular polarizer is not necessarily all the time as you need to know the best time period of the day to use it. For example in Singapore, the best time period to use Cir. PL in order to obtain a saturated blue sky is from 2:45pm to 4:30pm. Well of cuz it depends on that particular day's weather. If it's too cloudy, you can't get the desired results, but on the other hand the light that falls on other objects will be polarized to give a more vivd colors.
As for Cokin filters, you can use the 'P' system which consists of a filter holder (slot-in type) that holds up to 2 or more filters in combination without risk of vignetting. You just need to get a set of adaptor rings for yr respective lenses to fit different thread size. The Cokin 'P' system filters are made of special coated acrylic sheets and you can get a cheap small CD holder bag to store the filters. You need to wrap around the filters with a soft tissue paper or even microfiber cloth to prevent scratches and foggy mould/spots.
The Cokin P system is possibly the most widely used filter system in the world as it offers a near-perfect compromise between cost, versatility and image quality. A great benefit of this sytem is that polarizing filter rotates in its own narrow slot at the rear of the holder. This means you can use other filters, such as neutral-density graduates at the same time and control the position of both independently - aligning the graduate first, for example, then rotating the polarizer until the desired effect has been obtained.
In yr case, a ND2 with a polarizing filter should do the job with regards to TTL light metering to get the right exposure. I strongly recommend you a book, The Photographer's Guide to Filters by Lee Frost, a comprehensive guide to filters and how to use them. Suitable for photographers of all levels of expertise.