request.setEndPoint('https://login.salesforce.com/services/oauth/token');

request.setMethod('POST');

String reqbody = 'grant_type=password&client_id='+clientId+'&client_secret='+clientSecret+'&username='+username+'&password='+password;

request.setBody(reqbody);

Http p=new Http();

HttpResponse response=p.send(request);

result=response.getBody();

system.debug('@@@result = '+result);

return null;

<apex:pageBlockSection title="OAuth2.0 username & Password"
                          collapsible="false">


   <apex:pageBlockSectionItem >
        <apex:outputLabel value="ClientID"/>

        <apex:inputText value="{!ClientID}"/>

   </apex:pageBlockSectionItem>

   <apex:pageBlockSectionItem >
        <apex:outputLabel value="ClientSecret"/>

        <apex:inputText value="{!ClientSecret}"/>

   </apex:pageBlockSectionItem>


   <apex:pageBlockSectionItem >
        <apex:outputLabel value="UserName"/>

        <apex:inputText value="{!UserName}"/>

   </apex:pageBlockSectionItem>

   <apex:pageBlockSectionItem >
        <apex:outputLabel value="Password"/>

        <apex:inputText value="{!Password}"/>

   </apex:pageBlockSectionItem>


   </apex:pageBlockSection>

   <apex:pageBlockButtons >

     <apex:commandButton value="Get Access Token"
                         action="{!GetAccessToken}"
                         reRender="token"/>

   </apex:pageBlockButtons>

{!accesstoken}