Beware of color cast on cheap ND filters.
As for what ND filter, you can use this formula.
Most ND filters are labelled ND2, ND4, ND8 and so on. It means that your exposure needs to be 2, 4 or 8 times longer. The common ones for long exposure are ND400 and ND1000 from Hoya and B+W respectively.
Suppose your shutter speed is 1/200s. With ND2 it will be 1/200s * 2 = 1/100s. With ND1000 it would be 1/200s * 1000 = 2s. So you can work out what you need. Only you will know what aperture you prefer to shoot at and what kinda of shutter speed you normally work with. But usually even with a ND1000 you're not gonna get 30s in bright daylight unless you are using a tiny aperture.
cool! thks for educating me on the formula.. now i knw how to choose the nd filter to get..