So Xehanort locked Terra and he fall into the realm of Darkness and Aqua saved him so where did Ansem and Xemnas come from? Where did Xehanort's body go?

That's what DDD's for. They showed a glimpse of one of those scenes at the end of the last trailer.

Ansem SoD is Terranort/Apprentice Xehanort's Heartless and Xemnas is his Nobody. The Heartless is the person's heart after it gets swallowed up by darkness, and the Nobody is the empty shell of a body of a strong-hearted person who became a Heartless.

Xehanort took over Terra's body. It's presumed his own body disappeared and took refuge in Kingdom Hearts, as do all wandering hearts and bodies.

Afterwards, Aqua and Terra/Xehanort fought Aqua, resulting in Terra/Xehanort to fall into the realm of darkness. From here, Aqua saved Terra/Xehanort, during which time Terra was able to take a brief control and wipe out both of the two's memories. Unable to remember anything, Terra/Xehanort became Apprentice Xehanort and studied under Ansem The Wise. He was led by Braig to the side of darkness, with pieces of his memories returning to him slowly.

From there, there's no whole conformation, but there are bits we do know.

In the Ansem Reports (from KH1) we were told that Xehanort and the other apprentices lost their hearts and became Nobodies due to an accident of some kind. It's made out to seem like a science experiment gone wrong, by this point we know otherwise.

From a sneak peak of a trailer for KH3D, we saw that Xehanort actually attacked his fellow disciples and used his dark powers to turn them into heartless and nobodies. We don't know why though yet. It's possible that Xehanort tried to turn his disciples to Nobodies to form Organization XIII and create his own heartless. But it's also possible that the accident referred to in the reports was actually referring to Terra taking control over the Xehanort body and using his powers to take out the disciples and try to destroy himself, but only resulting in making a heartless and Nobody.

Dream Drop Distance gonna answer this question clearly

Here's how I know it.

 

MX needed a new "vessel" (a body) because he was getting old and needed to keep himself alive to see a new Keyblade War. He visited Eraqus and his pupils one day and saw that Terra had a great amount of untapped darkness in his heart. Only someone with that kind of darkness could withstand MX's plan.

 

After all of the BbS story, MX and Terra merged to become Terranort. After Aqua fought him and tried to awaken Terra's heart, MX tried to lock Terra's heart away inside him. This also locked his heart and caused the darkness in it to overflow. When the portal to the RoD opened and Aqua sent him back up, he had forgotten everything about himself except his "name", Xehanort.

 

Xehanort is Terranort without the memories of either Terra or MX. Xemnas and Ansem are the nobody and heartless (respectively) of Xehanort. We know now that when a person's heartless and nobody are defeated, in that order, the person themself comes back whole again. The problem with this is that only Xehanort himself comes back, though he now remembers that he can wield MX's keyblade (as seen in the 3D trailer) and he attacks Braig. This leads me to personally believe that Terra's consciousness is starting to take over, as only Terra would have a reason to hate Braig (the set up during his story in Birth by Sleep).

 

To answer your question, Xehanort ended up being found by Ansem the Wise and became his apprentice.

What they said since I was gonna say it.