A good way to start is to determine the melody. Write down the notes for the melody, since you probably already have the rhythm in your head
Now, since several songs have verse, verse, bridge, chorus (form A A B C) or some variation thereof, once you determine one part, you have a good chunk of the song.
For example, consider "Lord, I'm Available to You." It follows the plan A A B C. The two verses are "A", the bridge is "B" and the chorus, "Lord, I'm Available To You" is "C." When you get the melody for the first verse, you also have the main melody (with some variations) to the second verse. Learn these part cold, and write them down.
Now, work on bridges and fillers. These are generally climbing progressions, with dominant sevenths, augemented / diminished chords, etc. Write these down . . . you are almost home!
Finally, determine a good introduction and conclusion. Play the whole song and work on any rough areas until you are satisfied. Finally, deermine what key you are playing the song in so you can come back to that key later. Sometimes you need to play in every key so you can find whatever key the singer is in, that is completely true and you don't want to limit yourself by only playing in certain keys. But I am finding out that sometimes the singer is depending on the piano player to get them in the right key. And then you've got it :lol:
This is just an idea, everyone does it differently, so let God show you what works best for you. :lol:
Be Blessed . . . :lol:
BBoy