The lyrics of the song must follow the melody. As simple songs like "Mary Had A Little Lamb" do. And in general I start by adjusting the lyrics to "fit" the melody.

It is a very standard practice for me. I will drop a word, add a contraction, maybe drop an article like the words a, an, and it. Look for a two syllable word to replace a 3 syllable word. Understand others may write the lyrics and then find a melody for those lyrics. But, if you are comfortable with a melody you have written, and have some nice lyrics, do adjust one or the other or both until they are pleasing to your ear.

No real rules, what ever works for you, with practice you will be able to write melodies, and then put words to them. And also write words and put them to melodies. Or a combination of both. You don' know how it will end up until you try making the adjustments. Just plow through it until you are happy with the results.

"When you help somebody else you are really helping yourself"