I have been accused of molesting my girlfriend's teen daughter and a detective has been to my home to talk to me. Do I need to hire a lawyer?

Yes. Any sexual offense is very serious. If you are convicted, in addition to jail time, you will be required to register as a sex offender, perhaps for life. You should not speak to the police unless your lawyer advises it and is present with you. Anything you say to the detective can, and likely will be used against you. Even though you are under investigation and not charged as yet, a lawyer can be your conduit to the police and the District Attorney's office. Your lawyer can insist they stop contacting you for interviews since you are represented by counsel. He or she can also present information, if you have such, that would be helpful in convincing them a molestation did not occur. The jails are filled with people who thought if they could only tell their side of the story to the police, the police would see it their way. You have a right to remain silent for a reason. Use it. And seek out the best criminal defense attorney you can find in your area to represent you.