The most common trap that pop songwriters can fall into is thinking that they can simply sit down, scratch out some lyrics that mean nothing and turn it into a hit. Listen to the radio for inspiration and you'll realize that most successful pop songs have a theme, and that theme is present in each song from start to finish. If you change what you are singing about halfway through the tune, your listeners will be confused, and will most likely tune you out.
The structure of a song is just as important, if not more so, than the music and lyrics themselves. Most pop songs follow the "A A B A" formula, which most listeners are used to. The formula consists of two "A" sections that are relatively simple, such as your first two verses, followed by a "B" section that contrasts to the "A" section, and then finally a return to the "A" form at the end of the song. This is a tried-and-true formula, so if you are new to pop songwriting it's a good idea to stick to it at first. On the other hand, rules are made to be broken in the art world, so don't be too afraid to stray away from this system.
The entire notion of "pop" music is that it appeals to a mass audience. You should write pop music in a way that everyone can understand, so as to find the largest possible audience for your tune. Songs about things such as love, heartbreak, the "man" and parties are popular because most everyone who hears the song identifies with what is being said. Use these common human emotions and desires to craft a song that has the potential to appeal to a wide audience.
Writing a hit song is, above all else, about getting it stuck in some one's head. In order for this to happen, the melody of your song needs to play nicely with the lyrics you've written for it. Do a simple test to determine whether a song is easy to listen to and remember: Sing the main line of your song to a friend once, then ask her to sing it back to you. If she can do this easily, then you've got a good, catchy hook. If she stumbles over the words or forgets the words or the melody, you may want to revise one or the other.