Visualforce page:

<apex:page controller=”sample” id=”mypage”>
    <apex:form >           
       
        <apex:selectList id=”selected_list” value=”{!temp}” required=”false” size=”1″>
            <apex:selectOption itemvalue=”None” itemLabel=”–None–“/>
            <apex:selectOption itemvalue=”a” itemLabel=”a”/>
            <apex:selectOption itemvalue=”b” itemLabel=”b”/>
            <apex:actionSupport event=”onchange” reRender=”Details” action=”{!find}”/>
        </apex:selectList>
        <br/>
        <apex:panelGroup >       
            <apex:outputPanel id=”Details”>           
                The value based on select list value: {!temp1}           
            </apex:outputPanel>
        </apex:panelGroup>
        
    </apex:form>
</apex:page>

Apex controller:

public class sample
{
    public String temp {get; set;}
    public String temp1 {get; set;}

    public sample()
    {

    }
   
    public void find()
    {
        if(temp == ‘a’)
        {
            temp1 = ‘Welcome’;
        }
        else if(temp == ‘b’)
        {
            temp1 = ‘Thank you’;
        }
    }           
}


Output: