If you are working on the public Main Galaxy instance at http://usegalaxy,org. please see the taxonomy tools under the group "Metagenomic analyses". If you want to filter first, or cut columns, join datasets together, or otherwise customize your workflow, see the tools in the groups "Text Manipulation", "Filter and Sort", and "Join, Subtract, Group" - all work well on tabular datasets.
If on a local/local Galaxy, these same tools are in the Tool Shed and can be installed for use: http://usegalaxy,org/toolshed
If on another public Galaxy, the available tools can vary. Manipulation tools are generally standard, Metagenomic tools may or may not be included. If not, move to an alternative (Main, local or cloud).
Best, Jen, Galaxy team