If there are cancer types in view that you do not want to investigate, you will need to filter them out. Please see the help page 'How do I filter to just one cancer type' to get started with this.
Steps:
Add a column of data. Enter your gene or list of genes, select 'Gene Expression' and click done.
From the column menu at the top of the new column you created, select 'Chart & Statistics'
Choose 'Compare Subgroups'
Click the dropdown for 'Show data from' and choose your gene expression column.
Click the dropdown for 'Subgroup samples by' and choose the cancer type column.
Choose if you would like a box plot or violin plot and click 'Done'.