The Earth’s atmosphere probably had its origin in the gasses emitted by volcanoes; Earth was still very hot from its molten, impact-ridden origin, and volcanoes were abundant.
Water vapor is the most abundant component in volcanic gasses today, and this was probably the case in the early Earth as well. Ammonia, methane, and carbon dioxide are also volcanic gasses that were probably important in early Earth atmosphere.
Earth is able to retain its atmosphere because it is sufficiently large to have a gravitational force powerful enough to hang on to it, and because Earth has a magnetic field that protects it from the solar wind that would otherwise strip away the atmosphere.
The effects of not being large enough to hang onto an atmosphere can be seen on the Moon and Mercury.
For more on early Earth's atmosphere, follow this link.
Illustration is from here.
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