Define a summary field to compute the total of amount.
Create a new layout based on the table that you already have here.
Use Part setup to add a sub summary layout part. Specify that the sub summary part be visible "when sorted by" your Category field.
Put your new summary field into this new sub summary layout part.
If you want to see just the totals for each category and not the indivdual records that make up each category, delete the body layout part.
Return to browse mode and sort your records by the category field.
You can also add a second sub summary layout part, put the same fields in it, but specify the supplier field as the break (when sorted by) field.
If you sort your records by supplier, you see totals for each supplier. If you sort by category, you see totals for each category.