Out of all the possible genres to write in, this one is by far the hardest IMHO. An obstacle course with huge glops of Cheez-Whiz hidden behind every corner, just waiting to smother the listener in schmaltz
OK, now that I've got that off my chest.....
I suggest that you start with the lyrics before you ever play a note. I've had the best results by starting with stream-of-consciousness writing; simply letting the words flow freely and not worrying about rhyme, meter, or anything else besides setting down the raw emotion of what you're trying to express
If something hurt - write it down in as much detail as you dare. If something felt good - ditto. Was there a place that is associated with either of the two previous emotions? Describe it in minute detail. If there was/is a smell that triggers a memory or an unconscious reaction - describe it in detail. I could go on, but by now I think you've got the gist
Step 2 - Start playing around with those raw phrases and sentences, trying them out in various combinations.
Do not discard any of your raw stream-of-consciousness stuff under any circumstances. There may be times later on in the process when you write yourself into a corner, and it can be useful to go back to the original notes and restart in a slightly different direction. Do the word/phrase shuffling on a completely separate sheet of paper. As you continue to do this you will (hopefully) start to see patterns emerge
This is roughly the point where I pick up an instrument for the first time. I do all of my writing on an instrument that is capable of playing chords, normally a flat-top acoustic guitar. OTOH if your harmonic knowledge and chops are up to it, bass is perfectly capable of producing useable two or three note chords for the purpose. Even a single-note bassline can work well at times
In any case, look over your written output and home in on the phrases that have the strongest emotional power in the least amount of words. Don't worry about how other people might interpret the "message", that's not important - what is important is how
she will interpret that phrase. What you're trying to do here is provoke an involuntary emotional response, and the words that will do that are going to be specific to one particular person. Or, to put it another way, avoid generalities like the plague and send a message that only She can really understand
Step 3 - start trying out various chords and progressions with the Big Message phrase - better known in the trade as "The Hook". Sooner or later, you will arrive at something workable and a melody line will begin to take shape
Once you have the hook fairly well established, the rest of the verses are usually somewhat easier to construct. Again, start with your raw material and progressively refine it. If something triggers new raw material, write that down as fast as possible before you lose it completely
Step 4 - Time to start paying attention to structure. IME it is better to avoid strict rhyming as much as possible. Far better to rhyme the last word of entire verses than every other line, for instance. The more rigid the rhyming pattern, the more difficult it is the get the message across and the easier it is to lose the emotional impact. A certain amount of rhyming is a good thing - it helps you remember the lyrics when you're singing the tune, if nothing else - but too much can be a straitjacket that will force you into horrible clichés like Moon June Tune etc.
There's more (far more......) to the craft of songwriting than this, but this should be enough to get you started with some reasonable possibility of a decent result
IMHO it is well worth getting ahold of a rhyming dictionary and a book called "The Synonym Finder" (ISBN 0-87857-236-8). Both should be available at any decent-sized city library, or through inter-library loan if they're not on the stacks. Both have proven invaluable for getting me out of lyrical corners I've painted myself into
One last thing. All of my best love songs have ended up complete failures for their original purpose (to get The Girl) but turned out to be great successes with a larger audience. Life can be funny like that......
Good Luck, Kid. You're gonna need it