First copy base file from app\design\frontend\base\default\template\checkout\onepage\payment\methods.phtml
to your theme
app\design\frontend\your_packge\your_theme\template\checkout\onepage\payment\methods.phtml
After replace code from
<?php
$methods = $this->getMethods();
$oneMethod = count($methods) <= 1;
?>
<?php if (empty($methods)): ?>
<dt>
<?php echo $this->__('No Payment Methods') ?>
</dt>
<?php else:
foreach ($methods as $_method):
$_code = $_method->getCode();
?>
<dt id="dt_method_<?php echo $_code ?>">
<?php if(!$oneMethod): ?>
<input id="p_method_<?php echo $_code ?>" value="<?php echo $_code ?>" type="radio" name="payment[method]" title="<?php echo $this->escapeHtml($_method->getTitle()) ?>" onclick="payment.switchMethod('<?php echo $_code ?>')"<?php if($this->getSelectedMethodCode()==$_code): ?> checked="checked"<?php endif; ?> class="radio" />
<?php else: ?>
<span class="no-display"><input id="p_method_<?php echo $_code ?>" value="<?php echo $_code ?>" type="radio" name="payment[method]" checked="checked" class="radio" /></span>
<?php $oneMethod = $_code; ?>
<?php endif; ?>
<label for="p_method_<?php echo $_code ?>"><?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?> <?php echo $this->getMethodLabelAfterHtml($_method) ?></label>
</dt>
<?php if ($html = $this->getPaymentMethodFormHtml($_method)): ?>
<dd id="dd_method_<?php echo $_code ?>">
<?php echo $html; ?>
</dd>
<?php endif; ?>
<?php endforeach;
endif;
?>
<?php echo $this->getChildChildHtml('additional'); ?>
<script type="text/javascript">
//<![CDATA[
<?php echo $this->getChildChildHtml('scripts'); ?>
payment.init();
<?php if (is_string($oneMethod)): ?>
payment.switchMethod('<?php echo $oneMethod ?>');
<?php endif; ?>
//]]>
</script>
To
<?php
$methods = $this->getMethods();
$oneMethod = count($methods) <= 1;
?>
<?php if (empty($methods)): ?>
<dt>
<?php echo $this->__('No Payment Methods') ?>
</dt>
<?php else:
$p = 1;
foreach ($methods as $_method):
$_code = $_method->getCode();
?>
<dt id="dt_method_<?php echo $_code ?>">
<?php if(!$oneMethod): ?>
<input id="p_method_<?php echo $_code ?>" value="<?php echo $_code ?>" <?php if($p==1):?>checked="checked"<?php else:?> <?php endif;?>type="radio" name="payment[method]" title="<?php echo $this->escapeHtml($_method->getTitle()) ?>" onclick="payment.switchMethod('<?php echo $_code ?>')"<?php if($this->getSelectedMethodCode()==$_code): ?> checked="checked"<?php endif; ?> class="radio" />
<?php else: ?>
<span class="no-display"><input id="p_method_<?php echo $_code ?>" value="<?php echo $_code ?>" checked="checked" type="radio" name="payment[method]" checked="checked" class="radio" /></span>
<?php $oneMethod = $_code; ?>
<?php endif; ?>
<label for="p_method_<?php echo $_code ?>"><?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?> <?php echo $this->getMethodLabelAfterHtml($_method) ?></label>
</dt>
<?php if ($html = $this->getPaymentMethodFormHtml($_method)): ?>
<dd id="dd_method_<?php echo $_code ?>">
<?php echo $html; ?>
</dd>
<?php endif; ?>
<?php $p++; endforeach;
endif;
?>
<?php echo $this->getChildChildHtml('additional'); ?>
<script type="text/javascript">
//<![CDATA[
<?php echo $this->getChildChildHtml('scripts'); ?>
payment.init();
<?php if (is_string($oneMethod)): ?>
payment.switchMethod('<?php echo $oneMethod ?>');
<?php endif; ?>
//]]>
</script>