These days, with everyone having access to affordable recording gear, most publishers are accustomed to and expect to receive fully produced (some even want "master quality") recordings. Fully produced can mean many things - but the point is, a simply guitar/vocal or piano/vocal demo rarely cuts it anymore (unless that is the way you "hear" the song). If you are writing for other artists - you can perhaps get by with less production. If you want to place songs in film/TV then publishers will likely want something fully produced. Pick your battles very carefully.
If you have a publisher in mind (obviously in the genre you write best) - send a letter to the publisher to confirm they accept unsolicited material and ask what type of presentation they want. There is a book that is published every year "Songwriter's Market" - which identifies publishers, the genre they prefer, what type of package they want, etc.
If you have anything to include as a bio (press releases, reviews of gigs, etc) it does not hurt to include - but only if it can add real credibility. Call the publisher after a short time (say 1-2 weeks after sending the letter) ask if they recieved your letter. Be persistant, but be professional (and be nice). You will likely only talk to an assistant at first - but that person can be your best friend (your access to the publisher). If you are lucky enough to be allowed to send in a CD - normally they want no more than 3 songs - with your best song first.
Keep in mind - this is all about sales/marketing. You must first sell yourself before you can sell your song.
The absolute, most important thing - make certain you have the best vocalist you can find/afford to present your song. You may have the finest melodies and very clever, heartfelt lyrics - but if they are presented off key, etc. - you could blow an opportunity. If you are writing with a particular artist in mind - try to find a singer who sounds like that artist.
Lastly, make certain you are as good as you think you are - the competition is fierce and even the most succesful writers are only as good as their last song - so every day you could be competing with people who wrote hits for (name the artist or your preferred genre).
