It's a feature built in to Forge. I'm assuming you're using Forge? Make a new class that extends CreativeTab, register it, then use that custom tab when referring to the tab your items should display on.
public static final CreativeTabs tabName = new YOUR_TAB_FILE();
Instead of doing what demonic said, you can easily add this code (fetched from the FML wiki) to your @Init method:
LanguageRegistry.instance().addStringLocalization("itemGroup.tabCustom", "en_US", "My Custom Tab");
Replace the string "My custom tab" with your tab name, such as "My Mods' Ores" or something, and replace the tabCustom in the string "itemGroup.tabCustom" with your tab's name, such as "itemGroup.tabMymodsores". Hope this helps.