Title your song lyric. Sometimes the title itself can be a guiding tool in the direction of the lyrics. A strong title will also help you stay focused as you write the words to your song. Think of Willie Nelson's "On the Road Again." There's no doubt what the words to that song will be about. It isn't necessary to have a title before you begin writing, but if you can start with one, it can be helpful during the writing process.
Start with a simple structure that features two verses, a chorus, a verse, and a chorus. The verse of a song is the part of the song that presents the story of the song, typically in alternating lines of rhyme similar to a poem. The chorus is the part of a song between verses, usually with the same lyrics each time. As you progress, you may want to try this structure with a bridge after the final verse, just before the ending chorus. The bridge is typically a sudden change in the direction of the story or a slight variation on the chorus. It can be used to add variety to the structure.
Write words that are not difficult to pronounce. Big words or obscure words won't make your lyric memorable, except that people may remember it's difficult to sing along with your song. In many cases, people won't understand the lyric and will forget the song as soon as it's off the radio. Write with an audience singing in mind.
Write about subjects you feel strongly about or emotionally attached to when possible. You will no doubt write lyrics about love when you're not in love or lyrics about being down and out when you have hit records and a lot of money, but try to find some bit of emotional connection when you write these lyrics, even if you have to dig back into your past. If you're singing your songs and you can't connect with them emotionally, you can't expect your audience to connect.
Avoid cliche lyrics. It's not always possible to avoid some of the most common phrases used in songwriting because those are phrases that people like to hear. Again, listening to a lot of music will allow you to hear what's been done before. Tell stories in your songs. Use poetic language and devices. Make the words to your song stand out, but don't get pretentious. You want your audience to be able to relate to your song.