How to type visible text in an Acrobat document?

Hi there.I totally don't know how to use Acrobat and I'd like to know how to fill up an Acrobat document.
Whenever I try to type text in the blank fields, nothing shows and I can't see if there is text or what I'm writing.
It would be nice to know how to make the text appear in black.
I've tried the different tools but can't change anything.
Thanks for any help.