OK, so. First things first, a little bit of music theory never hurt anyone. You will be glad of learning a bit of music theory in the long run.
And by, bit of music theory, I just mean make sure you know what key you're writing in and have a reference that tells you what notes are in the key. (TRUST ME, this makes the whole song sound much better. Discordant notes are... creepy, and you don't want that in a sad or happy song.) I also had Piano lessons, and I also know that this method of composing probably does not work for everyone. ^^;
Now I'm gonna give you a very brief, probably unhelpful explanation of how I do things. However, this IS super long and I didn't intend it to be. OTL
The way I usually do things is I start in Anvil Studio (it's a free Midi Composer, very nice). I create the melody for my vocals with my lyrics first, in the very first track.
Then I add tracks with different melodies, but you want to listen to everything, to make sure it isn't clashing. Save often, as well. (Although who doesn't? XD //shot) I also like to add a rhythm track, basically drums. I export a midi-format 0 after I like how all the tracks are sounding. the midi-0 I exported is going to be imported into UTAU so I can make my UST.
After we're done exporting, it's time to head over to FL studios. Open it up, and import your regular midi (this would be what you saved you song in Anvil Studio as). Now, you can select different instruments for your tracks. (If you don't like any of FL Studios VSTs, DSK Instruments has a lot of great free ones.) Then, once you like what you hear, you can export as an MP3 (512-point-sinc is the highest quality, I recommend it even though it will take a bit longer). Best to title it as MySongName Off-Vocal.
Then I do my UST thing. Import the midi-0 file into UTAU (the regular midi save file that Anvil Studio uses doesn't appear to work with UTAU, hence we exported the Midi-0 which does work. IDK why.) Input the lyrics, do your tuning with the UTAU you want to sing the song, and render project wav. (I can't remember what the thing is actually called off the top of my head.)
And then we go back to FL Studio, mix the damn thing, and export that MP3 (there are tutorials on mixing in FL studio, so I'm not gonna go into that. OTL) This time, you should title it MySongName ft MyUTAU.
Aaand excluding art and pv stuff, you're done!
...You know what, I should probably make a tutorial on dA or YouTube. This probably didn't help you at all. OTL
Helpful Links:
All Natural Major Keys with Notes:
http://musictheorysite.com/major-scales/list-of-all-major-scales
All Natural Minor Keys with Notes:
http://musictheorysite.com/minor-scales/list-of-all-minor-scales
This handy thing tells you the characteristics of the keys, like happy, sad, dismal, etc:
http://www.wmich.edu/mus-theo/courses/keys.html
DSK Instruments (you'll thank me later, this site is a life-saver and my go-to when I want new instruments):
http://www.dskmusic.com/
Anvil Studio:
http://www.anvilstudio.com/