Pick your topic. Most pop songs range very little in subject matter. Write about how much you love someone, how much someone loves you, how much everyone loves you, how much money you have (unless it's not a lot--that'd just be depressing), the jewels in your teeth, a dance move or about partying.
Use unique or made up language. It's good to bring new ways of expressing oneself to the culture and pop songs are some of the best places to introduce new words. Look at urbandictionary.com for inspiration (see Resources below).
Wear sunglasses. This may not have a lot to do with writing a pop song, but you'd be hard pressed to find a pop star that doesn't wear shades. Some things you just can't question.
Have a gimmick. The gimmick of the song is what sets it apart from the other songs. It could be something like yelling "What," like Lil John, or adding "izzle" after the first letter of words, like E-40 and Snoop Dog. The gimmick is best placed in the chorus of the song.