Songs are frequently generated by specific ideas, rather than overarching ones. With that in mind, it is important to keep a daily journal in which you note conversations, observations and memories. Eventually, you will be able to see connections and themes. You will also be able to revisit the journal for inspiration when you need it.
Sometimes the best way to develop new song ideas is by listening to the music you love. Play a part of the music yourself, and see where it takes you. This is an excellent way to move into your own work. Later, rearrange the introduction to the piece so it complements the song you have created.
Focus on one particular memory--your childhood home, a walk you took in a different city, a book or poem you have read. Think about the mood of the memory, and try to capture this mood in a melody. If the mood changes, adjust the melody accordingly. When you are satisfied with melody, experiment with different lyrics over the music. It is a good idea to keep all drafts of your work.
Approaching songwriting by using essential chord progressions is a favorite for musicians. Start with something simple such as the following: C Ddim/F C Ddim/F C/E Am Dm G C. Improvise, and make it your own.