public class AccountController {

  public Contact con { get; set; }
  public String selectedaccountId { get; set; }
  public List<SelectOption> getListOfAccounts() {
       List<Account> AccountList = [select id,Name from Account] ;
       List<SelectOption> AccountOptionList = new List<SelectOption>();
       AccountOptionList .add(new SelectOption( ' ' ,'---Select---'));

       for(Account acc : AccountList )
       {
                  AccountOptionList.add(new SelectOption(acc.id , acc.Name));
       }
      return AccountOptionList ;
}

public PageReference loadDetails(){
    con = [Select LastName, FirstName From Contact Where Id = :selectedaccountId ];
    return null;
 }
}

<apex:page controller="AccountController">
   <apex:form >
      <apex:pageBlock >
           <apex:pageBlockSection title="Select the Account" >
              <apex:OutputPanel >
                <apex:selectList value="{!selectedaccountId}" size="1" multiselect="false"  >
                  <apex:selectOptions value="{!ListOfAccounts}" />
                  <apex:actionSupport event="onchange"
                    action="{!loadDetails}"
                    rerender="contactDetails"/>

               </apex:selectList>
          </apex:OutputPanel>

      </apex:pageBlockSection>
      <apex:pageBlockSection title="Details" columns="1" id="contactDetails">
             <apex:outputField value="{!con.firstname}"/>
                <apex:outputField value="{!con.lastname}"/>
               </apex:pageBlockSection>
  </apex:pageBlock>
</apex:form>