There are many, many Hubs in HubPages (some are quite excellent) that can teach you about reading and writing music. Start by reading them. Daniel Carter has written and published a great deal of music. He is one you might check first.
A lot of music writing software (Cakewalk for certain and probably Finale and Sibelius) can interface with a music keyboard, so that you can play on the keyboard and the music is recorded in written form. [You may also be able to use the computer keyboard to write the music - different letter keys representing different notes. It takes more explanation than I can give here.] The cost of the software varies. I'm a big fan of Cakewalk Home Studio, but it also has some drawbacks.
Also, have you considered recording (on tape or digitally) your music and then hiring someone else to write it out? There is a highly published musician I've heard about who doesn't know how to read or write out her own music; she plays and sings it, and she has an assistant who writes it out in notation.
Since you're "broke," the idea of hiring someone probably sounds ludicrous and impossible. But you might be able to find someone who would do it for free initially, with the promise of future payment if your song sells. That sort of arrangement if often offered on Craigslist.