How can I ask the Selenium-WebDriver to wait for few seconds in Java?

Well, there are two types of wait: explicit and implicit wait.
The idea of explicit wait is

WebDriverWait.until(condition-that-finds-the-element);

The concept of implicit wait is

driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);

You can get difference in details here.

In such situations I’d prefer using explicit wait (fluentWait in particular):

public WebElement fluentWait(final By locator) {
    Wait<WebDriver> wait = new FluentWait<WebDriver>(driver)
            .withTimeout(30, TimeUnit.SECONDS)
            .pollingEvery(5, TimeUnit.SECONDS)
            .ignoring(NoSuchElementException.class);

    WebElement foo = wait.until(new Function<WebDriver, WebElement>() {
        public WebElement apply(WebDriver driver) {
            return driver.findElement(locator);
        }
    });

    return  foo;
};

fluentWait function returns your found web element.
From the documentation on fluentWait:
An implementation of the Wait interface that may have its timeout and polling interval configured on the fly.
Each FluentWait instance defines the maximum amount of time to wait for a condition, as well as the frequency with which to check the condition. Furthermore, the user may configure the wait to ignore specific types of exceptions whilst waiting, such as NoSuchElementExceptions when searching for an element on the page.
Details you can get here

Usage of fluentWait in your case be the following:

WebElement textbox = fluentWait(By.id("textbox"));

This approach IMHO better as you do not know exactly how much time to wait and in polling interval you can set arbitrary timevalue which element presence will be verified through .
Regards.