Gene Expression Threshold

Hello,

I am a high school student working on a research project identifying the expression of relevant human cancer related genes in populus trichocarpa. I would like to display the gene expression values I have using a heatmap, but I am unsure of how to determine the threshold for over expressed/under expressed(do I need to standardize?) and if there is a different way I can display my results. Any suggestions or tips would be deeply appreciated.

Thank you