My baby is 6 weeks old and already has skin rashes. What can I use?

Be more specific. Rashes of various types are fairly common in babies, but no one can suggest a treatment without knowing the cause. Your best bet is to have your baby examined by your pediatrician or a dermatologist.
Rash. At this age it would be important to figure out why is your baby getting rashes already. Is it the milk your baby is drinking? What soaps, lotions and detergents do you use for baby? I would recommend not to use perfumed soaps, lotions, or detergents. If that doesnt help see your baby's doctor.