You need to know the main 3 domains, the kingdoms with in, and the phylums. I could be wrong, but from what I have seen looking at the practice tests, it looks like they don't ask anything really beyond the phylum level. However I suppose they could ask you to identify the phylum or kingdom of a common organism in that phylum/kindgom, although I don't specifically remember seeing any questions like that.
Here are some examples that I remember:
-One question you had to know that fungi is not photosynthetic
-one question you had to know what bryophytes are (liverworts, hornworts, and mosses) to answer the question
-it seems like they ask about cnidarians and echinodermata a lot.
I would say know the domains, know the kingdoms, know the important phylums. Have an idea of the main features of phylum (symetry, nutrition method, circulation, respiration, reproduction, etc) and know a few examples of things you are familiar with from each phylum. For example, if you are asked about echinodermata, and know what a sea urchin looks like--that it has radial symetry, you should be able to answer any question about echinodermata involving symetry even if they ask you about an organism you are not familar with such as sea lilies.