I started my own law practice in October of 2006. I left my GC job hoping my contacts at that time would be sufficient to ensure a steady flow of work.
Before taking this leap, I was in-house counsel for nearly 9 years at two different companies. Before that, I was a law firm associate for two-and-a-half years (following a two-year federal judicial clerkship). So, at the time I launched my solo practice, my understanding of what a private firm practice should be was quite dated – particularly with respect to legal research.
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 are more likely to be admitted.