Hi.
Even though you can learn a lot from going to school... this is an art so you don't really NEED to do that.
if you don't have singers availible, write down the words that you want to be sung. So you at least have lyrics.
Do you have a keyboard? LIke a musical one (yamaha, casio, whatever) or just a basic piano? You can try playing them out there... it may not be the FULL song, but you get an idea of what you want to do.
There are some pretty cheap and free software that you can get that will let you create music.
FL Studio has a free demo that won't let you save a project, BUT you can export an MP3 or wave of whatever you make.
If you google around, I konw there are some free ones, but it's a little late and I can't quite remember what they are at the moment, sorry. Google something like "Free DAW" or "Freeware music creation software" and you should find a bunch of matches.
With these you can sort of plot out the melody and drums of the song you want to make.
Again, it may not sound EXACTLY how you want, but you will have worked out a road map.
There are a bunch of things that go into music production, but take it slowly and don't expect to make something that sounds like it came off some big pro game or movie... I bet the people who made those songs didn't start out sounding quite like that either.
I hope that was some help.