Almost all work, especially for newcomers, is by the audition process. The client will audition people of a type that think most appropriate (age, sex, tone, accent, etc.) by sending their script to talent agents who will then get the scripts to those on their roster they think most appropriate. The agent will record at his / her studio, or talents will record on their own, and email the finished, edited performance to be added to the compilation with others.
My agent floored me recently when she told me that according to her research, the numbers are staggering: involving an aggregate of as many as 1000 auditions for ONE job. "How is that possible?" I asked, as I pointed a finger gun to my head. She said ad agencies have interns narrow it down, making the first cut, removing anyone clearly out of range. Then the project goes up the ladder at the ad agency, as first junior, then senior levels will cull the 'possibles' till they end up with three choices, who are then presented to the client.
Obviously, the odds are hard to beat. And you must know that beyond those who clearly can not do the work or do not fit the desired criteria, it's all subjective.
Here's what you need:
A great demo. This is a recording which shows off your talent and flexibility – for example, maybe 45 seconds of you reading pieces of fully produced scripts (real jobs or demos) which show your vocal range, ability to interpret, emote, sell, etc. (My agent reminded me: you really only get 8 to 10 seconds to make an impression.) If you do not, they will leave you for the next person. best work first.
A source of audit material. There are services (you pay) online which feed you scripts which are being cast, to demo and send back. Agents can also provide such opportunity (for which they will take a fee from each job you do get.)
A place to record your demos and auditions. Since it usually requires many trials before you land a job, you need a high quality 'studio.' You can record on many programs in your computer – you will need a microphone, a microphone pre-amplifier, a sound card, speakers or headphone, and probably some room treatment to prevent the hollow room sound from marking you as an 'amateur.'
You have to be immediately commanding, or likeable, believable, or whatever else is called for.
With the odds you face through the amount of competition, you have to do everything you can to market yourself, and to generate the confidence to keep trying.
Right place right time? It's all subjective at some point. Never underestimate luck.