I have a Nikon lens NIKON AF-S 24-85 F/3.5-4.5G ED VR. And I'd like to get an ND.10 filter.
What's the right filter size for that lens? And what brand would you suggest
The NIKON AF-S 24-85 F/3.5-4.5G ED VR lens has a filter thread size of 72mm. It's listed in the specs for that lens at Nikon USA.
If you have other lenses that you may also be interested in using with filters in the future, you might also consider a filter holder system such as those offered by Cokin or Lee.
Such systems are particularly an advantage, even with only a single lens, if you ever decide to use graduated ND filters. The square/rectangular filters those systems use can be rotated and adjusted up and down to move the line between the darker and lighter sides of the filter to match the composition of your scene.
Another advantage is that you only need to buy one of each type of filter, and then only buy a single (relatively cheap) adapter ring for each size lens you have.