As long as you are still in the Newbie rank, you cannot change your signature. To use a signature and complete it with personal text, you must at least rank up to Junior Member. Lack of knowledge makes you ask questions like this, you can read the explanation shared (_act_) on how to add or edit a signature on your account profile.
Don't rush, my friend, currently you are still in the Newbie rank, so don't be surprised if your profile doesn't have space to enter a signature code, personal text and you can't use an Avatar. To achieve it you need to raise the account rank first, you can use Avatar when the account is already in the Full Member rank. So be patient my friend, enjoy the process first, you can use all of that when the time comes.