Buddhism has a long monastic tradition, accommodating both monks and nuns. Buddhist nuns, known as "bhikkhuni," live a life of study, meditation and sometimes service to communities. They must take eight precepts, including a vow of celibacy. They strive for full liberation from suffering, for themselves and, in some traditions, also the suffering of others.
Decide what type of Buddhism suits you best. If you are considering becoming a nun, you may already have made this decision. There are many forms of Buddhism, including its Chinese, South Asian and Indian forms, such as Mahayana and Theravada Buddhism. Traditions differ in their beliefs, meditation techniques and practices.
Try to gain some experience of Buddhism before ordination, if you have not already. You are unlikely to be accepted for ordination without such understanding. Start attending Buddhist classes, and try meditating consistently for a period. One way of gaining a monastic experience without being ordained is to go on a meditation retreat. At these you will observe similar precepts to monastic ones, and engage in meditation.
Find a monastery that suits you when you are sure you want to try living the monastic lifestyle in earnest. There is no limit to where you can go. If you are interested in Theravada Buddhism, for example, you may wish to travel to Southeast Asia. However, there may be communities within your country of origin. There are many forms of ordained communities, including modern forms in Western nations. For example, the New Kadampa Tradition opens centres in communities and places a strong emphasis on outreach and running classes for communities. The Triratna community in the U.K. has a broader interpretation of ordination, and some ordinands do not take a vow of celibacy. They work in businesses, have families and go on retreats.
Apply to live at the monastery or community. Monasteries will not ordain you immediately, but will require a trial period. This is also beneficial to you, to see if you wish to commit to this way of life.
Accept your ordination when it is offered to you. This will involve taking eight precepts and taking refuge in the "triple gem:" the Buddha, the Sangha (community) and the Dharma (the truth or teaching). You will usually shave your head and be given robes to wear.