Goto your module etc/system.xml and you can add  simply the following code in system.xml file

<fields>
<your_tag_name translate="label tooltip comment">
<label> - Select - </label>
<frontend_type>select</frontend_type>
<source_model>adminhtml/system_config_source_yesno</source_model>
<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>
<comment>Write comment.</comment>
</your_tag_name>