Embarrassing but...
It was one of the earlier seasons of America's Best Dance Crew. They would use "Just Dance" at the end of every episode when a team was eliminated and I liked the song, so I tried to find it on iTunes but I never could... because for the longest, I thought it was Ashlee Simpson singing it.
After a while, I just gave up trying to look for the song until later that year when the song became more popular and I figured out what the name of the artist was. I slowly started getting into her, buying The Fame on iTunes a few songs at a time for a period of months until I had the whole album. When the Paparazzi video came out, that's when I really started liking her.