<apex:page standardController="Task" extensions="LogACallControllerExtension">
  <apex:sectionHeader title="Log a Call" />
    <apex:form id="pageForm">
      <apex:pageBlock title="Task Edit" mode="edit">
        <apex:pageBlockButtons location="both">
           <apex:commandButton value="Save" action="{!save}" />
           <apex:commandButton value="Cancel" action="{!cancel}" />
        </apex:pageBlockButtons>
        <apex:pageBlockSection title="Call Information" columns="2" collapsible="false">
           <apex:inputField value="{!Task.OwnerId}"/>
           <apex:outputField value="{!Task.WhatId}"/>
           <apex:inputField value="{!Task.Subject}"/>
           <apex:outputField value="{!Task.Status}"></apex:outputField>
           <apex:inputField value="{!Task.ActivityDate}"/>
           <apex:inputfield value="{!Task.Activity_Type__c}"required="true"/>
           <apex:inputfield value="{!Task.Description}"style="width:400px;height:100px;"/>
        </apex:pageBlockSection>
      </apex:pageBlock>
    </apex:form>
</apex:page>

public class LogACallControllerExtension {
public Task task {get; set;}
    public LogACallControllerExtension(ApexPages.StandardController controller) {
            this.task = (Task)controller.getRecord();
            task.whoId = ApexPages.currentPage().getParameters().get('id');
            task.activitydate = Date.today();
            task.Status='Completed';
            task.Activity_Type__c='NA';
            task.IsReminderSet=true;
            task.ReminderDateTime=System.now();
    }
}