What sort of effect are you looking for and what camera and lenses do you have?
Also what is your budget?
There are many different filters
E. G circular
Polariser
Square
ND grad
ND
Etc
You can also spend from say £10 to hundreds of pounds for one filter.
Square filters come in several different sizes E. G. 85mm, 100mm. The size you need will depend on your lenses.
You need to provide further details to enable people to provide a meaningful answer