Mrthdrb

Just like there is a default value for 'State/Province' field which is: 'Please select region, state or province' I need to add a default value to City field in billing and shipping steps in checkout.
In my case the City is drop-down list getting its list of cities from database.

In app>design>frontend>base>default>template>checkout>onepage>billing.phtml, the city field code is:

<div class="field">
 <label for="billing:city" class="required"><em>*</em><?php echo $this->__('City') ?>
 </label>

 <div class="input-box">
 <input type="text" title="<?php echo $this->__('City') ?>" 
 name="billing[city]" 
 value="<?php echo $this->escapeHtml($this->getAddress()->getCity()) ?>" 
 class="input-text <?php echo $this->helper('customer/address')->getAttributeValidationClass('city') ?>"
 id="billing:city"/>
 </div>
</div>

If I understand you, you want to force your users to have default value on city section then change this:

value="<?php echo "your city name"; //$this->escapeHtml($this->getAddress()->getCity()) ?>" 

This will make default city on city field.

PS

By default Magento pools city data for registered customer from database using $this->escapeHtml($this->getAddress()->getCity()) function.

So when you change this, it will never pool that data.

If like the state/province you just want to show what is effectively help text, I suggest you add a placeholder attribute to the input box. So something like...

<<input type="text" title="<?php echo $this->__('City') ?>" 
 name="billing[city]" 
 value="<?php echo $this->escapeHtml($this->getAddress()->getCity()) ?>" 
 class="input-text <?php echo $this->helper('customer/address')->getAttributeValidationClass('city') ?>"
 id="billing:city"
 placeholder="<?php echo $this->__('Please enter city'); ?>"/>

Then if they empty the value out the placeholder text will appear again.

Regards,
Andy