Help with Excel if/then statements

I should start by saying by I'm only a casual used of Excel. I'm creating a household budget and I want to list expenses as they are incurred, with category.

I would like to total the amount spent in each category, but I'm unsure of how to write the formula and haven't found much online. More than likely, I'm not searching with the right terms.

Column C has the amount spent (A is date, B is what the money went toward) and column D has the category name. I have categories like housing, food, auto, etc. in a drop-down list.

Ideally, at the bottom of each week's column I'll have each category listed with the total spent.

Help? Thanks!!