How do I sum up specific things in Numbers? I have a spreadsheet in the newest iWork Numbers.app, with something like:

CATEGORY | POINTS  
      HW | 100
      HW | 100
      NW | 12
      HW | 50
      NW | 23


I want to just sum up all the Category = HW into a cell: I'm not sure how. I think I need to use the SUMIF() function but I get stuck afterwards.
You are correct in that you need to use the SUMIF function.
This is the specification from the Function Browser:

SUMIF(test-values, condition, sum-values)


Try this in your cell:

=SUMIF(CATEGORY; "=HW"; POINTS)