One of the most common ways to get someone's attention in Japanese would be to use the te-form of the verb 聞く (kiku // to listen), which is 聞いて (kiite), which literally translates to something like "Listen (to me)."
Note: Be very careful with the following phrases, because they differ quite significantly depending on gender.
(Literally: "Hey, listen.")
How am I supposed to know what's feminine language?!
This is actually one of those things that you'll pick up naturally over time hearing lots of different guys and girls speaking Japanese.
However, here's one free tip:
Elongated ね (ne) sounds at the beginning of a phrase are almost always feminine. Usually these will be written like ねえ (nee) or ね～ (ne~). Be especially careful of the ね～ version, because this ～ mark indicates that the tone of the speakers voice goes down and then up, holding out the え sound at the end of the word. Very feminine.
Double ねえ sounds very feminine, too. Just to be safe, most guys should just avoid saying ねえねえ in front of any sentence... although you'll hear a lot of girls do this when getting someone's attentions.
Saying ねえねえ before the sentence above would give it a more excited nuance:
nee nee, kiite
Hey, guess what?!!
Males tend to use なあ (naa) instead.
naa, kiite kure yo
Hey, guess what?
So feel free to throw those two sentences in front of just about any (good or exciting) news that you'd like to tell someone.
Saying "Wanna hear something cool?" in Japanese
The phrase that I want to introduce here is not actually all that close to "Wanna hear something cool?" If I wanted to say that phrase exactly in Japanese, I'd probably say:
ii koto shiritai
Wanna hear something cool?
(Literally: "Want to know something good?")
That doesn't really have the nuance that the speaker is holding some unknown, (almost secret) information, which is what we have in English's "Guess what?" For that, we might say...
ii koto oshiete ageru
I'll let you in on a little secret.
(Literally: "I will (do you a favor and) teach you something good.")
There is a very significant, though subtle difference between the phrase above and one that ends with ～ようか (you ka):
ii koto oshiete ageyou ka
Wanna hear something cool?
(Literally: "How about I teach you something good?")
This second phrase has more of the sense "Wanna hear something cool?" Whereas the first one sounds a little snobbish, almost like, "I think I'll give you a good bit of information."
Fretting About Nuances and Perfect Phrasing
...is not worth your time. I know that these articles talk a lot about very tiny differences in Japanese, but that's mostly because I think that it's very useful for understanding Japanese to hear explanation of subtle differences. Still: You don't need to worry about small stuff when communicating in Japanese.
It took me a long time to get a sense for these differences, and I still mess them up all the time. Part of the beauty of learning a language is noticing when you've started to get a sense for its subtleties, nuances. You start to notice when someone's being little rude to you, maybe underestimating your language ability. You start to understand when someone is telling you something... without telling you something.
If all of this seems intimidating, just don't worry about it.
You'll get there. One day at a time.
p.s. If you want to learn more Japanese, I recommend NativShark.