system.xml
<?xml version="1.0" encoding="UTF-8"?>
    <config>
       <sections>
            <mysection translate="label" module="mymodule">
                <label>My Section</label>
                <tab>catalog</tab>
                <frontend_type>text</frontend_type>
                <sort_order>110</sort_order>
                <show_in_default>1</show_in_default>
                <show_in_website>1</show_in_website>
                <show_in_store>1</show_in_store>
                <groups>
                    <mygroup translate="label" module="mymodule">
                        <label>My Group</label>
                        <frontend_type>text</frontend_type>
                        <sort_order>99</sort_order>
                        <show_in_default>1</show_in_default>
                        <show_in_website>1</show_in_website>
                        <show_in_store>1</show_in_store>
                        <fields>
                            <myfield translate="label comment">
                                <label>My Field</label>
                                <frontend_type>text</frontend_type>
                                <sort_order>1</sort_order>
                                <show_in_default>1</show_in_default>
                                <show_in_website>1</show_in_website>
                                <show_in_store>1</show_in_store>
                            </myfield>
                        </fields>
                    </mygroup>
                </groups>
            </mysection>
        </sections>
    </config>


Now, to add default value to the admin configuration options, you need to write the following in config.xml file of your module. The should be inside config node.

config.xml
<default>
    <mysection>
        <mygroup>
            <myfield>My Default Value</myfield>
        </mygroup>
    </mysection>
</default>

http://www.magentocommerce.com/wiki/5_-_modules_and_development/admin/xml_structure_for_admin_configurations

system.xml
<?xml version="1.0" encoding="UTF-8"?>
    <config>
       <sections>
            <mysection translate="label" module="mymodule">
                <label>My Section</label>
                <tab>catalog</tab>
                <frontend_type>text</frontend_type>
                <sort_order>110</sort_order>
                <show_in_default>1</show_in_default>
                <show_in_website>1</show_in_website>
                <show_in_store>1</show_in_store>
                <groups>
                    <mygroup translate="label" module="mymodule">
                        <label>My Group</label>
                        <frontend_type>text</frontend_type>
                        <sort_order>99</sort_order>
                        <show_in_default>1</show_in_default>
                        <show_in_website>1</show_in_website>
                        <show_in_store>1</show_in_store>
                        <fields>
                            <myfield translate="label comment">
                                <label>My Field</label>
                                <frontend_type>text</frontend_type>
                                <sort_order>1</sort_order>
                                <show_in_default>1</show_in_default>
                                <show_in_website>1</show_in_website>
                                <show_in_store>1</show_in_store>
                            </myfield>
                        </fields>
                    </mygroup>
                </groups>
            </mysection>
        </sections>
    </config>


Now, to add default value to the admin configuration options, you need to write the following in config.xml file of your module. The should be inside config node.

config.xml
<default>
    <mysection>
        <mygroup>
            <myfield>My Default Value</myfield>
        </mygroup>
    </mysection>
</default>

http://blog.chapagain.com.np/magento-add-default-value-to-system-configuration-option-fields/

This article shows how to add default value to any system configuration option fields. System configuration option fields means those options under System -> Configuration.

Suppose, you have the following system.xml file. System.xml file is necessary to add admin configuration options.

Magento Add default value to System Configuration Option fields

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