Strictly speaking, you need not be married in order to adopt a child.
In reality, most people regard a married couple as more likely to provide the child with a conducive environment as there would be both male and female role models for the child.
Nonetheless, there are many situations where a single person has successfully adopted a child if he or she meets the basic criteria. A single man however, cannot adopt a female child, unless under very special circumstances.
Even if you are married, you can still apply to adopt a child on your own and not jointly with your spouse provided your spouse consents to it.