Getting ideas for writing your song.

Some tips when you're not "inspired".

 You won't always wake up every day with a brilliant song in your head, most of time you'll have to work hard to get somewhere. These are some  methods you can use.

• JAM!

Very obvious but jam until you bleed if necessary. Sometimes you'll get something right away, sometimes this will take hours and even sometimes you won't have a thing.
 Be ruthless with your ideas, keep only those with value and work with them until you feel you have "something".
 Try different approaches of the same thing(different rhythm, extra/less notes for example) to ensure you're not missing a better idea.
 Make sure you're in  mood to play, otherwise everything will sound "meh"(This happens way too often).

• Hum or whistle

 Great way to brainstorm, you can do it anywhere, anytime. Very useful for chord progressions and melodies. Want to improve your humming?. Beat Box.
 You can have a portable drum to work while humming. A basic kick and snare sound it's all you need. 
 I personally make heavy metal and beat boxing helps me a lot. I can set a tempo and write drum patterns  really easy and quick, even sometimes I do it while playing guitar (instead of humming) to make a good jam session.

• Listen your favorite bands/artist/composer

 Before jamming listen a few songs from a band that you like to inspire yourself, this doesn't mean you have to redo the song, you just want a little push to start working.
 Influences are very important, in fact everybody begin by imitating their idols, later you develop your own style, your own sound. 
 Be careful about cryptomnesia happens when you think you did something original but is not, you heard it before but you can't remember it. More info here: 
http://en.wikipedia.org/wiki/Cryptomnesia.