I want my textfield to stand out, because users can work with the rest of the page only after they filled it in. The interface is clean and strict, so I can probably add some color to grab attention. Do you have any suggestions?
I would suggest fading out the rest of the form partially until the user has filled out the required field. This makes it clear that there's more for the user to do on that page, but that the first field (the only one that's shown at full opacity) is the only thing they can interact with yet.
This also allows you to avoid cluttering up the form with extra colors or highlights that may distract the user from the form content itself.
Though highlighting as this example below will draw attention to your text field,it is not a very good way since it would just stand out and would disturb the overall flow.
Since you want to allow other users to enter data only after that text field has been filled, you could enable your form validation logic to be such that the other fields are disabled until that field is filled correctly.
Another alternative approach is to hide the other fields until this field is entered correctly.
Another alternative option is to just get your page to focus on the required text field on page load and highlight it.
Any UI control can have the input focus. You can give the input focus to your textbox explicitly when your page is loaded. This way, the user will already see the blinking caret there. You can see this behavior on google.com
Also, most UI controls have a disabled state. When disabled, they are usually grayed out and cannot receive the input focus even if the user clicks on them. If you disable all controls except the first when your page loads, this will also be quite clear. This is very similar to a previous answer (fading out the form), but I think it's a bit better to use the existing disabled state instead of implementing your own using the fade..