As Andy says you need to find the filter ring size.
You've got multiple choices with 10 stops, screw in or drop in.
Screw in are OK, cheaper option and less outlay on kit, but you can't easily use an ND grad (even if you've slowed your exposure down by 10 stops there might still be a wide variation in brightness from the sky to the ground) and taking them on and off is a bit of a faff (you'll be needing to remove them to recompose, meter and focus before putting them back on for the exposure).
Drop in filters are a better solution but involve carrying and buying more gear.
Your choices, depending on what focal lengths you want to cover, are the 85mm P series size filters or 100mm filters like Lee, Cokin Z Pro and Hitech 100. For UWA on your 500d you'll want 100mm, for standard kit lenses etc you will get away with 85mm.
If you go down the slot in route you will need a filter holder. P series copies at least can be bought for a few pounds on ebay, or more for genuine Cokin or Hitech ones. Cokin Z Pro, Hitech or Lee tend to be more expensive but you can use them on wider angle lenses.
Then you'll need an adaptor ring for the lenses you intend on using it for.
Finally you'll need the filter itself. Hitech do 85mm and 100mm, Lee do the Big Stopper but it is £100 or so and on months back order.
