Jeremih knows all about getting his cake and eating it too—just read the lyrics to his lead single, "Birthday Sex."

Over a dawdling, piano-based beat, the 21-year-old singer/songwriter/producer croons, "You say you want passion/I think you found it/Get ready for action/Don't be astounded."

The song "is actually based on a birthday story of mine," says Jeremih (last name: Felton). "You'd think someone would've written a song like that already—one that caters to females on their day. It's the perfect hit."

Sitting at No. 54 on Billboard's Hot R&B/Hip-Hop Songs chart just four weeks after its debut, "Birthday Sex" is indeed proving to be a hit. But Jeremih admits he didn't anticipate the catchy tune would be such a sensation.

"I didn't even think it would be the first single," the Chicago native says. But then his manager, Louis Duran, played a few of Jeremih's tracks for Barbara A. McDowell, music director of the city's adult R&B station WPWX.

"We were actually pushing another song called 'My Ride,' " Jeremih says. "But she heard 'Birthday Sex' and said she could play that around a Beyoncé or Kanye track. She basically picked the first single."

Jeremih—who got his start playing drums, piano, bass and saxophone—only discovered singing three years ago. And now he's recording a self-titled debut album after signing with Def Jam just a month ago.

"We took a meeting with [Island Def Jam chairman Antonio] 'L.A.' Reid and [executive VP of A&R] Karen Kwak, who had heard 'Birthday Sex,' " Jeremih says. "She wanted to know what I sounded like live because a lot of people think I use Auto-Tune." That same day, Jeremih became part of the Def Jam family.

Described as "urban pop," Jeremih's album is slated for release June 30. The set is being produced by Mick Schultz and doesn't feature any guest collaborations. Among the recorded tracks are "Runway," inspired by Tyra Banks' TV show "America's Next Top Model," and "Starting All Over," which draws inspiration from Stevie Wonder.

Although the label's promotional campaign is in the preliminary stage, a collaboration with MySpace is in the works. "Birthday Sex" also is available on iTunes and imeem for downloading. It has sold 16,000 digital copies, according to Nielsen SoundScan.

It's all pretty heady stuff for someone who never thought of himself as a singer. "It wasn't until I performed a song I'd written before an audience in college that I realized I could even do it," Jeremih says. "The feedback made me feel like a star."