Please write a Psalm from your own heart and experience. It does not need to rhyme or have flowery words. It needs to speak your truth about what you feel about God, either as a believer in God or as an unbeliever. Think about the Psalms you have read, learn from them. Notice has similar many of them are. Maybe your will be similar to them. The short biblical Psalm is 6 verses (each verse with two lines), so your Psalm should be at least that long.