New to marine tanks, need some help...

[Fishward]

filter sock is pretty much just a sock of some sort (some people use pantyhose)(official ones available at your LFS) that your tank drain will flow through in your sump. catches all the big chunks, fish poop, extra food, etc.

skimmer should be based on total water volume. so if you have a 65 gallon tank, 40 gal sump (25 gallons water) minus 10-15 gallons of space for rock, you'll want a skimmer rated for at least 80 gallons, 100 would be nice.