Question:
How do I improve my songwriting?
Answer:
The best way is to constantly and persistently study what hit songwriters do. Learn from the best. Listen to the radio and take notes on what hit songs have in common. What is their structure like? Do they end verses on major or minor chords? Do they have a bridge? How many bars are in the intro?

Question:
What if I just write lyrics?
Answer:
Frankly, it's exceptionally rare that someone in the music business asks, "Can you find me a lyricist - quick?!" Not to say that somebody who is strong with melodies might not look for a collaborator whose strength is lyrics. But it is very rare that a record label would hunt down somebody who just writes lyrics.

Question:
Could working with another songwriter improve my chances of success?
Answer:
Yes, in at least two ways. Number one: Two heads are often better than one. You just might write a better song with a collaborator. Number two: If you're really lucky, the person you co-write with may be more advanced and/or better connected than you are, giving you a leg up on getting your songs cut.

For more: http://www.taxi.com/faq/songwriting/#q1