Magento : Add configuration setting for custom module

Consider module Bd_Demo and model Test, then
Create System.xml in your module demo/etc
<?xml version="1.0"?>
<config>
    <tabs>
        <demo module="demo" translate="label">
            <label>Tab name</label>
            <sort_order>100</sort_order>
        </demo>
    </tabs>
  <sections>
    <demosection module="demo" translate="label">
      <label>Settings</label>
      <tab>demo</tab>
      <frontend_type>text</frontend_type>
      <sort_order>1000</sort_order>
      <show_in_default>1</show_in_default>
      <show_in_website>1</show_in_website>
      <show_in_store>1</show_in_store>
      <groups>
        <demogroup translate="label">
          <label>External Datatbase</label>
          <frontend_type>text</frontend_type>
          <sort_order>100</sort_order>
          <show_in_default>1</show_in_default>
          <show_in_website>1</show_in_website>
          <show_in_store>1</show_in_store>
          <fields>
            <!-- New fields go here -->
         <demofield translate="label comment">
              <label>Test Name</label>
              <comment>
                <![CDATA[This text appears just beneath the field with a small arrow.
                <span class="notice">It can contain HTML formatting too!</span>]]>
              </comment>
              <frontend_type>text</frontend_type>
              <sort_order>10</sort_order>
              <show_in_default>1</show_in_default>
              <show_in_website>1</show_in_website>
              <show_in_store>1</show_in_store>
            </demofield>    

          </fields>
        </demogroup>
      </groups>
    </demosection>
  </sections>
</config>

Then Define ACL in config.xml
<config>
    <adminhtml>
        <acl>
            <resources>
                <all>
                <title>Allow Everything</title>
                </all>
            <admin>
                <children>
                    <system>
                            <children>
                                <config>
                                    <children>
                                        <demosection>
                                            <title>Demo name</title>
                                                <children>
                                                    <demogroup>
                                                        <title>Settings</title>
                                                        <sort_order>1</sort_order>
                                                    </demogroup>
                                                   
                                                </children>
                                        </<demosection>
                                    </children>
                                </config>
                            </children>
                                </system>
                </children>
            </admin>
            </resources>
        </acl>
    </adminhtml>
</config> 

Also we access the value of this filed  as
Mage::getStoreConfig('demosection/demogroup/demofield ');

Also we can define the default value of this filed in config.xml
<config>
<default>
        <demosection>
            <demogroup>
                  <model>demo/test</model>                 
                  <demofield>Default Value</demofield>
            </demogroup>
        </demosection>
</default>
</config>