My wife already gave birth to a baby. Although more than one month has passed the bleeding continues. I want to make sex with my wife. What should I do in this case?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is prohibited to have sex with your wife as long as the confinement blood has not stopped during the confinement period which is 40 days. But, if the blood stops before she finishes the 40 days, then she makes Ghusl and prays and you can have sex with her.
If the blood stops after 40 days, the woman makes Ghusl and her prayer then is valid, so is her fasting and there is no restriction on her husband as far as having sex with her is concerned even if some yellow liquid or other vaginal secretions are noticed.
Abu Dawood reported from Umm Atiyya (Radiya Allahu Anha) that she said: "After we became pure, we did not consider the yellowish or muddy discharge to be anything" .
If the blood however continues after 40 days, the woman has to identify whether it is similar to the normal menstruation blood which she had before she became pregnant or different from it.
If it is similar to it, then it is menstruation blood and she has to refrain from doing things that are forbidden during the menstruation period, i.e. praying, fasting and having sex until the period of her normal menstruation ends. If the blood continues after that period, it is then considered Istihada blood which does not prohibit praying fasting or having sex. In a case of Istihada, one has to perform Wudu for every prayer.
Allah knows best.