As someone who has been arrested for your first crime, you may not think it's particularly important to defend yourself.
That's a mistake.
If you don't defend yourself, you could face bias in court, or you could be treated unfairly. For this reason, it's a good idea to work with a criminal defense attorney. A good defense can help you reduce the penalties against you -- and could even help you escape a conviction altogether.
Why fight charges if you committed the crime?
When you're accused of a crime, it's almost always a good idea to mount a defense. Your attorney will take the time to look into how you were arrested and how you've been charged. It may be clear that there were mistakes in the investigation, or your attorney may be able to find out that the evidence isn't adequate or has gone missing. In those cases, it might be possible to have the charges against you dismissed.
If your attorney finds that there is a good case against you, then they can look to see what other types of defenses you may be entitled to use. In some circumstances, you may be guilty of a crime but not really deserving of punishment. For example, an assault charge may have resulted from an act of self-defense. A theft charge could result from a simple misunderstanding. Developing a narrative that helps the court better understand what really happened -- and why you shouldn't be punished -- can be a help in reducing the likelihood of a conviction.
Your attorney can also help you obtain a plea agreement if that's appropriate in your case. A plea agreement can often reduce the severity of penalties you might otherwise receive after a trial and conviction.