ok i'll join in here with some advice, having a few different filters, here are what i consider the basics
1. 77MM, THIS IS VITAL, GET EVERY FILTER YOU PURCHASE IN 77MM, the reason for this is because filters are anywhere from $25-$400 EACH. step up rings are about $6 each, and since if you ever get into fast glass, most of those lenses are 72 or 77mm, you can always make a 77mm filter fit a 58mm lens but you cannot make a small filter larger
2. there are several great makes out there, tiffen, hoya, B&W, singh-ray, etc. i can't see much of a difference in the straight nd, now i have and use both straight nd and a very expensive variable from singh-ray. that singh-ray variable is a great filter, but it takes some practice to get used to, and is a $400 lifetime purchase
3. if you think you might carry and use a variety of different strengths of filter, save up those dollars and get a kit, these usually end up saving a few $ per filter, come in a nice pouch, and allow a nice across the board range. i did not get a kit when i started to get into fireworks, only because i already had the singh-ray. B&H has a couple of nice packages, and don't forget to use the click through link from here, every little bit helps defray the cost of bandwidth.
link below to b&h for a nice ranged tiffen kit, showing this am as temporarily out of stock, but i'm sure they will have it soon, REMEMBER, always come back to the site here, then click through so the sale shows up for tim
CLICK HERE