A siege charm is a scroll that you use. You purchase them from ivory tower. You use it like a buff scroll. Next time you die, on a siege field only, you will not loose the 1%. When you die, you have to another scroll. So grab a good 10-15 of them if you plan on a good day of sieging.
Cost varies by grade, and the grade of the scroll you need depends on your level, so go take a walk to ivory tower and check it out :) But generally I'd say roughly 50k should be an average guesstimate. It starts much cheaper, but I wouldn't recommend going to a siege if you're level 20 or 40 anyways ;)
You can "Go" to a Seige grounds, and attack people on the grounds, and they can attack you. But you can't do anything seige related if you are not in a clan that is signed up for the seige, meaning there is no way you can sneak in and take the castle.
If you want to watch the seiges, there are cameras set up, and you can usually catch a fair bit of the action watching the seiges from the viewing crystals. They are cheap, and no chance at death. If you are fairly low level, they may also give you a better view, as there is probably no way you would get to a vantage spot near the gate without accidently/purposly being killed
If you die, you lose 1/4 of what you usually lose, while on the seige grounds. You can buy the scrolls, as mentioned earlier, but be warned, once your higher leveled, the scrolls are fairly expensive.