public without sharing class MyClass {

    public String username{get;set;}
    public String password{get;set;}
    public PageReference logado {get;set;}

    public pageReference myMethod(){
        logado = Site.login(username, password, null);
        if(logado != null){
            // code
        }
    }
}

/**
* An Apex class that creates a portal user
*/
public class SiteRegisterController {
    // PORTAL_ACCOUNT_ID is the account on which the contact will be created on
    //  and then enabled as a portal user.
    //Enter the account ID in place of <portal_account_id> below.
    private static Id PORTAL_ACCOUNT_ID = '<portal_account_id>';
    public SiteRegisterController () {
    }
    public String username {get; set;}
    public String email {get; set;}
    public String password {get; set {password = value == null ? value : value.trim(); } }
    public String confirmPassword {get; set { confirmPassword =
        value == null ? value : value.trim(); } }
    public String communityNickname {get; set { communityNickname =
        value == null ? value : value.trim(); } }
    private boolean isValidPassword() {
        return password == confirmPassword;
    }
    public PageReference registerUser() {
        // If password is null, a random password is sent to the user
        if (!isValidPassword()) {
            ApexPages.Message msg = new ApexPages.Message(ApexPages.Severity.ERROR,
                                                          Label.site.passwords_dont_match);
            ApexPages.addMessage(msg);
            return null;
        }
        User u = new User();
        u.Username = username;
        u.Email = email;
        u.CommunityNickname = communityNickname;
        String accountId = PORTAL_ACCOUNT_ID;
        // lastName is a required field on user, but if it isn't specified,
        the code uses the username
            String userId = Site.createPortalUser(u, accountId, password);
        if (userId != null) {
            if (password != null && password.length() > 1) {
                return Site.login(username, password, null);
            }
            else {
                PageReference page = System.Page.SiteRegisterConfirm;
                page.setRedirect(true);
                return page;
            }
        }
        return null;
    }
}

/**
* Test class.
*/
@isTest
private class SiteRegisterControllerTest {
    // Test method for verifying the positive test case
    static testMethod void testRegistration() {
        SiteRegisterController controller = new SiteRegisterController();
        controller.username = 'test@force.com';
        controller.email = 'test@force.com';
        controller.communityNickname = 'test';
        // registerUser always returns null when the page isn't accessed as a guest user
        System.assert(controller.registerUser() == null);
        controller.password = 'abcd1234';
        controller.confirmPassword = 'abcd123';
        System.assert(controller.registerUser() == null);
    }
}