In its most basic form a song is "melody" (music) and "lyrics" (words) as these are the only things that can protected under copyright
If you can whistle or hum a tune and write lyrics you have enough to start songwriting
Depending on your music theory knowledge and whether you can play an instrument or not will dictate how far beyond this you can go - as well as access to recording gear (hardware and software) and knowledge of production etc. etc.
You have started with some lyrics so let's start there...
The pain inside has I look into her eyes........
I walk out the door knowing she doesn't want me any more....
My heart beats.....completing one of life's defeats
Just hurt and pain nothing ever to gain
Let's assume this is your chorus, which could form the central theme/message of you song
It would make sense if you chorus contained the title of the song - and it is common for the title to be an interesting and engaging little phrase that often appears in the first or last line of the chorus - or is repeated within it
In your case the titles that jump out to me are:
"The Pain Inside", "Hurt & Pain", "Nothing Ever To Gain"
A good lyric writing tip is to try to "clean up" your lyric to make it "tighter"
Do you really need the words "just" and "ever"? (Only you as the writer can decide for sure)
If you were to ditch "ever" - then "Nothing To Gain" sounds like a great title to me
Now that you have written your chorus you are ready to write your verses - so that they "set up" your chorus
Eg - why is there pain inside? Why doesn't she want you any more? What would you have gained?
It is also a good idea to try to make the lyrics about the listener to make it a bit more personal to help them "engage" with the song a bit more
Eg - "as I look in to
YOUR eyes"
This is much more engaging and I feel more involved as the song is now about "me"
I don't care about your relationships with someone else
Some people say that you should have the word "you" within the first 20secs of your lyric
There are loads more "tips n tricks" but hopefully this will get you started
What music knowledge do you have - do you play an instrument?