To filter your inventory click the funnel icon located on the inventory menu (Press I). The menu that appears will be titled Pack Tab Settings. Click Create New Tab a new line will be created named 'New Tab' click the name column on the new line that was just create and give it a name.
Next select the types of items you'd like to apply to this tab that will be visible when this tab is active in your inventory. Finally click apply. On a side note if you wish to have a tab automatically created for each category click auto-fill Tabs and a tab will be create along the top of your inventory for each category.