In US: must be an attorney licensed and in good standing in any state, territory or DC.
Outside US: must be a lawyer or equivalent (eg counselor, barrister, advocate, solicitor), duly educated and licensed/accredited and in good standing.
As a general rule, experienced and currently practicing lawyers, and those teaching law in the legal academy, are more likely to be admitted.
Excerpts from The Unique Psychological World of Lawyers:
In his book, Authentic Happiness (Free Press, 2002), Dr. Seligman reviewed his research on whether any personality attributes were consistently correlated to success in any of 104 careers he studied. Interestingly enough, the only career he found consistent correlations for was lawyering. And the attribute? Pessimism. Pessimism was so highly correlated with success in lawyers that the higher the pessimism in law students, the higher their grades.