1. Choose topic.
Before writing your first song, make sure to have an idea in your head on what to write about. If you have a plan, it will be easier for you to know which direction you should lead. For example, If you choose your main topic about love, you will know already what kind of lyrics you need to compose, what kind of melody and etc. Just give yourself an inspiration!
2. Structure your song.
For beginners, it's good to know how many verses you would like to sing and in what order. If you listen to some pop song, you definitely will notice some alike style of writing the structure of the lyrics. Often it has something as Verse 1 - Pre-Chorus, Chorus - Verse 2 - Pre-Chorus - Chorus - Bridge - Chorus. It's a basic structure of songs, so you can choose it for your first creation! Or else you can check out different sources, like this one link
3. Find the main melody.
A song that has an interesting melody, which can be as chord progression or strumming notes, has a lot of potential. So, do not neglect this step and try to compose an interesting melody. You would likely to start with a chord progression, and you also can choose the most popular ones for your first song, for example C Major - G Major - A Minor - F Major. You can also find various chords on the internet.
Writing your first lyrics can be a struggle for beginners, but with practice you will start rhythm the words better and better! I would really recommend to analyze the lyrics of your favorite artists. Try to understand, which kind of rhythm they use, how they combine words. It will all help you to understand that lyrics can be different, and you will see a lot of ways of how to match words together. Just keep writing and writing, the more you compose, the more knowledge you get.
These were my 4 tips on how to write your first song! Always mind that there's no room for music, you can do whatever it feels right for you. Observe different artists, analyze their art, and try to compose your own! It is okay if you will be not satisfied with the outcome, but as I said earlier, the more you compose, the more knowledge you get. Good luck with your first art!