Display Categories and SubCategories in Magento

Put This code in phtml file.
 

1) Display Top Level Categories Only In Magento

<?php $_helper = Mage::helper('catalog/category') ?>
<?php $_categories = $_helper->getStoreCategories() ?>
<?php if (count($_categories) > 0): ?>
    <ul>
        <?php foreach($_categories as $_category): ?>
            <li>
                <a href="<?php echo $_helper->getCategoryUrl($_category) ?>">
                    <?php echo $_category->getName() ?>
                </a>
            </li>
        <?php endforeach; ?>
    </ul>
<?php endif; ?>

2) Display Top Level Categories and ALL Subcategories In Magento

<?php $_helper = Mage::helper('catalog/category') ?>
<?php $_categories = $_helper->getStoreCategories() ?>
<?php $currentCategory = Mage::registry('current_category') ?>
<?php if (count($_categories) > 0): ?>
    <ul>
        <?php foreach($_categories as $_category): ?>
            <li>
                <a href="<?php echo $_helper->getCategoryUrl($_category) ?>">
                    <?php echo $_category->getName() ?>
                </a>
                <?php $_category = Mage::getModel('catalog/category')->load($_category->getId()) ?>
                <?php $_subcategories = $_category->getChildrenCategories() ?>
                <?php if (count($_subcategories) > 0): ?>
                    <ul>
                        <?php foreach($_subcategories as $_subcategory): ?>
                            <li>
                                <a href="<?php echo $_helper->getCategoryUrl($_subcategory) ?>">
                                    <?php echo $_subcategory->getName() ?>
                                </a>
                            </li>
                        <?php endforeach; ?>
                    </ul>
                <?php endif; ?>
            </li>
        <?php endforeach; ?>
    </ul>
<?php endif; ?>

3) Display Top Level Categories and Current Categories SubCategories In Magento

<?php $_helper = Mage::helper('catalog/category') ?>
<?php $_categories = $_helper->getStoreCategories() ?>
<?php $currentCategory = Mage::registry('current_category') ?>
<?php if (count($_categories) > 0): ?>
    <ul>
        <?php foreach($_categories as $_category): ?>
            <li>
                <a href="<?php echo $_helper->getCategoryUrl($_category) ?>">
                    <?php echo $_category->getName() ?>
                </a>
                <?php if ($currentCategory && $currentCategory->getId() == $_category->getId()): ?>
                    <?php $_category = Mage::getModel('catalog/category')->load($_category->getId()) ?>
                    <?php $_subcategories = $_category->getChildrenCategories() ?>
                    <?php if (count($_subcategories) > 0): ?>
                        <ul>
                            <?php foreach($_subcategories as $_subcategory): ?>
                                <li>
                                    <a href="<?php echo $_helper->getCategoryUrl($_subcategory) ?>">
                                        <?php echo $_subcategory->getName() ?>
                                    </a>
                                </li>
                            <?php endforeach; ?>
                        </ul>
                    <?php endif; ?>
                <?php endif; ?>
            </li>
        <?php endforeach; ?>
    </ul>
<?php endif; ?>