This action is very easy to use:

Use as a Head Action.

That's it.

The first textfield in the form will automatically be selected. The visitor will not have to click in textfield to be able to write. The visitor can just start writing.

This action does exactly the same with a little differ.

Use as a Head Action.

If you have one form only in the page, you don't need to change anything. But if you for some reason need to select another Text Field, than the first one, you write in the name of the Text Field. Or if you have more than one form you can write in the Form name and Text Field name.
If you know what you are doing, you can enter numbers instead of names.

(Remember JavaScripting start counting from 0 (Zero).