How can we have a baby, my wife being HIV positive?

Seek high risk OB. You can have a baby if your wife is HIV positive. First see a high risk OB doctor for consultation.
An HIV neg baby. Make sure you find a good high-risk gyn first...One who deals with HIV routinely. Second..Make sure she adheres to treatment and is undetectable. They may recommend a c-section at the end. They may also use IV HIV meds at the time of birth. The child may also temporarily be placed on meds by mouth.