I can learn music by listening to recordings, but as I may be unsure of what they are singing, I'd like to have lyrics in front of me.
Any songs will do- Bollywood, pop, folk

try exploring the web for sites with lyrics, you may find links to foriegn lyrics too

to sing, it is suffecient that u have a lyrics. it dosent matter in what language it is written, unless u know reading it. as you have asked this question in english, i do not feel that u will have any problem with english transliteration.

it is very difficult to find lyrics in hindi script. u can get abundantly in english for the same hindi song. just type the name of the film/song in google and u will get plenty of them.

if u still persist on hindi lyrics........ u should intensively search on the net. may be a rare chance that you get one.

best of luck!!!

check here-
http://www.bollywoodlyrics.com
http://www.hindilyrix.com/
http://www.cs.wisc.edu/~navin/india/songs/
http://www.bollywoodblitz.com/lyrics/index.php
http://www.indian-music.com/Lyrics/
http://www.geetmanjusha.com/hindi/