While a number of companies will perform various types of genetic testing, it is difficult to make sense of the tests and put them into useful context. It is best to speak with a genetic counselor to help interpret the results and risks and can help you with family planning if a risk is determined.
If you are worried about a medical condition, speak to your child's doctor and ask for a referral. If you have different concerns, there are independent DNA labs.
The simplest way is to submit a blood sample to a laboratory that is qualified to carry out genetic screening procedures. Unless you are a qualified doctor or health professional, you should not attempt to do it yourself. DNA analysis can also reveal information from other samples such as your baby's hair.