A few details about the cartoon are below:
- A boy was transported to another world and met a girl.
- Together they searched for some disks via a locket the girl had.
- The discs could be converted into little monsters.

*** help..

Sounds like the anime series,.

Monster Rancher (99)
a.k.a. Enbanseki No Himitsu

Genki's a kid obsessed with rollerblading and his favorite dueling game
Monster Rancher. He so good, he wins a special version of the game
And finds out that the Monster Rancher world is all too real. He turns
On his game for the first time, and eagerly waits to start playing. Meanwhile,
In the Monster Rancher dimension, a similar disk is activated at he same time,
And it pulls Genki through a portal into a world where all the monsters
Of the game actually exist.

Yeah definitely sound like an anime. I love anime, I'll have to check it out.

Seem to be Monster Rancher, Very good-looking.