It helps if you specify a specific question, or at least show us the error you are receiving.

<apex:page Controller="Logincls" sidebar="false">
    <apex:form>
        <apex:pageBlock title="GMAIL ACCOUNT">
            <apex:pageBlockSection title="ENTER USERNAME&PASSWORD">
                <apex:inputtext label="USERNAME" Value="{! UserID}" />
                <apex:outputText></apex:outputText>
                <apex:inputtext label="PASSWORD" Value="{! Password}"/>
            </apex:pageBlockSection>
            <apex:pageblocksection>
                <apex:commandButton value="LOGIN" action="{!login}" /></apex:pageblocksection>
        </apex:pageBlock>
    </apex:form>
</apex:page>

apex:

public class logincls {
   // public string pass; //why have this as a class variable ?
    public string userid {get;set;}
    public string password {get;set;} 
    public pagereference login {
            string pass = [select password__c from login__c where user_id__c = : userid]; //may want to encrypt passwords, or actually not store them at all, specially if you deal with other services like gmail, there are better authentication mechanisms for this.
            if (password == pass){
                pagereference hello = new pagereference('apex/helloworld'); 
                hello.setredirect(true); 
                return hello; 
            } else{
                return null;
            }
}

 Only typed it here, so may have typos or may have looked over somethings, but this should get you in the right direction.