Do COG functional annotation, at RAST seed annotation server. you will get GO numbers.

@Das , As you said you have Trinity.fasta you can do annotation with the Trinity utility Trinotate (https://trinotate.github.io/) . you can do annotation bu using BLAST2GO or Bingo App of Cytoscape.

I think that the best way to get the GO terms for your Trinity.fasta is to follow the Trinotate protocol (https://trinotate.github.io). At the end you will have a report with the GO terms and much more useful information.