What things should be keep in mind while deploying project or single program from Sandbox to Production environment ?

I have created one program & it is executing fine in sandboox environment but it is running in production environment & showing me error of code test coverage is of 15% & it must be atleast 75%.

The code is as follow...

<!-- Page: -->
<apex:page controller="sampleCon">
    <apex:form>
        <apex:selectRadio value="{!country}">
            <apex:selectOptions value="{!items}"/>
            </apex:selectRadio><p/>
            <apex:commandButton value="Test" action="{!test}" rerender="out" status="status"/>
     </apex:form>
     <apex:outputPanel id="out">
          <apex:actionstatus id="status" startText="testing..."> 
             <apex:facet name="stop"> 
               <apex:outputPanel> 
                  <p>You have selected:</p> 
                 <apex:outputText value="{!country}"/> 
              </apex:outputPanel> 
            </apex:facet> 
          </apex:actionstatus> 
     </apex:outputPanel> 
</apex:page>
            
/*** Controller ***/
public class sampleCon {
    String country = null;
         
    public PageReference test() {
        return null;
    }
                
    public List<SelectOption> getItems() {
        List<SelectOption> options = new List<SelectOption>(); 
        options.add(new SelectOption('US','US')); 
        options.add(new SelectOption('CANADA','Canada')); 
        options.add(new SelectOption('MEXICO','Mexico')); return options; 
    }
                   
    public String getCountry() {
        return country;
    }
                    
    public void setCountry(String country) { this.country = country; }
}

 Now there are two files & Controller : SampleCon & other is VFPage : RadioButton...I uploaded both the files from outbound change set of sandbox environment & I deployed it from inbound change set of production environment but getting test coverage & action gets failed.

Please help me...