Make text stay within box

[Craray]

Hi everyone,
I just started out using Webdesigner MX, and while I (think) am making a lot of progress (I have some dreamweaver background) there's one "simple" thing which I can't seem to figure out.
When I make a textbox and preview this in a browser in which the text are shown bigger (often with mobile phones, but also regular pc users) the text just ignores the boundaries and instead of wrapping it goes on.
Is there a simple way for the text to stay within the box?
(In the image below I used a normal textbox as an example)

Thanks in advance! (...and my apologies for possible grammatical errors as I'm Belgian)