Well, it really depends what kind of "spoiler" you want, after all, there are two of them:
- netabare ネタバレ
Spoiler which spoils a story.
(you probably want this one)
- spoiraa スポイラー
Spoiler of a car which makes it go faster, like in Initial D or something.
(this word is a gairaigo 外来語 and not the word you want)
Anyway, if you're talking about the "spoilers" I think you're talking about, you want the word netabare ネタバレ, not spoiraa スポイラー, and in that case you actually want two different words: neta ネタ and bare バレ.
- neta ネタ
Material for a news story.
Content of a story.
- bareru バレる
To be found out.
To be exposed.
Just like in a phrase such as himitsu ga bareta 秘密がバレた, "the secret was exposed," when we're talking about the contents of the is story it is neta ga bareta ネタがバレた, "the contents were exposed" or something like that, which is often shortened as just the noun netabare ネタバレ, meaning "spoiler."
Spoiler Alerts in Japanese
Knowing that netabare means "spoiler" it's very easy to make phrases that mean "do not spoil" or "no spoilers" in Japanese. See:
- netabare shinaide ネタバレしないで
Do not spoil.
- netabare shinai de kudasai ネタバレしないでください
Please do not spoil.
- netabare wa dame ネタバレはダメ
Spoiling is no good! (meaning of dame)
- netabare ki wo tsukete ネタバレ気を付けて
netabare niwa ki wo tsukete ネタバレには気を付けて
Beware of spoilers.
There are some words which aren't exactly neta but are similar. In particular, nakami 中身 and naiyou 内容, both have the meaning of "contents."
While neta would be more like what makes the story fun or important plot points, nakami is literally "inside" and you could use to say "don't tell me what's inside," for example. The word naiyou, on the other hand, is used to say the "contents" of a magazine, for example, but in its entirety. It's about all the text, all the contents, not specific important points.
Other Uses for Neta
Because neta ネタ means "contents" or "material" it can be a little confusing in how it's used. Basically, any idea you use to make something is a neta, therefore a thing is made out of neta.
For example, the phrase neta ga nai ネタがない means "there's no material" and it's often used by people writing blogs (like me) when they are out of stuff to write about (like me) and start writing about the most inane bullshit (like how to say spoilers in Japanese, seriously, gimme neta).
On the other hand, if you look at the website Pixiv, they actually have a tag for neta ga oosugite tagu ni komaru e ネタが多すぎてタグに困る絵, "pictures with too much material troubled by the tags."
This happens to exist because you can tag pictures in Pixiv, but only up to 10 tags, so some poor folks have crammed up so much neta into a single drawing that 10 tags just won't do it. (seriously, you only need to draw 11 different characters for that to happen. It's not even that difficult. It doesn't even need to be a single drawing.)