How did Sparrow know Turner was necessary?

He can't know that Old Bill was "killed"...

Jack found out what happened after the fact and was able to put two and two together.

The chronology of events is as follows:

• Jack is the captain of the Black Pearl
• Jack knows where to find Cortez' treasure
• Barbossa mutinies and becomes the captain, Jack is marooned
• Barbossa leads the Black Pearl to Cortez' treasure
• Bootstrap Bill feels guilty over what they did to Jack
• The pirates realize they've been cursed
• Bill gives away part of the treasure (to Will) as a way to punish Barbossa and his crew by making it impossible to put all of the treasure back and thus lift the curse
• Barbossa drowns Bill by tying a cannon to his bootstraps

Jack can make a reasonable guess that Bill partook in the share of Cortez' treasure and thus is cursed as well.

Bill would've been stripped of valuables before it happened (scavenging off the dead is common), so any of the cursed coins would've been found and returned to Cortez' treasure, which clearly hasn't happened since the curse is not lifted.

Bill has a family on land. If Bill has a cursed coin and it wasn't on his person when he was being executed, Bill's son (Will) is going to either knowingly own it, unknowingly have stored it with his dad's possessions, or will have known where he spent the coin.
In all of these cases, Will, as Bill's only son, is the best lead anyone has to the missing cursed coin.

The thing is, how did Jack know Turner's blood was necessary to lift the curse knowing that he was marooned by Barbossa before the crew got cursed?

If you're asking how Jack knew that blood of the thieves was required: his knowledge about the curse's existence most likely entails knowing how to lift the curse. Clearly, the other pirates knew as well, so it's not that big of a secret.

If you're asking how Jack could know that the child of a dead pirate suffices, that is unclear. He does know Tia Dalma so he may have enough knowledge of the occult to make an educated guess.
Also keep in mind he is the son of captain Teague, the keeper of the code of the Brethren Court, which can adequately explain how Jack is in the know on general pirate knowledge such as a highly sought after legendary treasure and the story surrounding it.

Or he didn't know for sure but confidently assumed it anyway (it's in character for Jack Sparrow). The rules of magic aren't always scientifically verifiable and you just have to take them at face value.

All pirates (including Barbossa - thinking Elizabeth was Bill's child) seemingly know for a fact that the blood of a dead pirate's child suffices to complete the ritual. How they know it never explained, and shouldn't really be questioned too closely as it's an arbitrary magical rule anyway.

According to the story told by Mr. Gibbs to Will later in the movie, Jack was the Captain of Black Pearl and he was betrayed by his mates after he has given the location of the treasure.

Also we learn in 3rd movie that Bootstrap Bill Turner did not participate in mutiny and gave medallion to his son for which he was thrown overboard with canon strapped to his boots.

Jack had already heard all these stories over the years but got confirmation of the curse when shipmates from Black Pearl attacked Port Royal and entered the jail where Jack was imprisoned.

After that Jack was smart to know that Black Pearl and it's crew were not in Port Royal for any treasure as they already posses a huge one and cannot be killed. He knew that there was some other purpose.

Earlier when Jack gets into fight with Will his face seemed to him to which he even says,

You look familiar ... Have I ever threatened you before?

Only when he learns his name he is sure that Barbarossa was there in search of Will to end the curse.

Recall the scene in Curse of the Black Pearl when Jack saves Elizabeth and takes her ashore. Jack takes note of the Pirate medallion and says

Jack : Where did you get that?

Now, Jack knows that all of Aztec gold is not with Barbossa and his crew. But, he's still not sure how Elizabeth got it.

Then comes the attack at Port Royal by Barbossa. Jack gets the confirmation about the curse from a Barbossa's crew man - Koehler.

Jack : So, there is a curse? That's interesting.

Koehler: You know nothing of hell.

Jack Sparrow: That’s very interesting.

Now, Jack knows that Barbossa is here for the medallion but still not sure why and how Elizabeth has it.

Scene III, Elizabeth is kidnapped. Will comes to Jack asking for help. Jack finds that Will loves Elizabeth and wants to save her. And finally, when he knows that he's William Turner, things start making sense for Jack. Jack knows it's Bootstrap's medallion. Will had it. Elizabeth took it. They are in love. Barbossa kidnapped Elizabeth assuming she's a Turner ( Though Elizabeth herself said she's Elizabeth Turner thinking that would protect her ). So, Jack knows the curse won't be lifted with Elizabeth's blood, so he has a chance to negotiate with Barbossa and regain the Black Pearl.