The first thing I want to tell you is know your craft. In other words, be prepared with all your might. When I first started, I had very little training, some here and there but I made it my business to sponge everything up and get around people with experience and knowledge. I wanted to learn so much that it hurt. You have to have passion for real acting. Not just for getting noticed or being in the limelight. Those are very weak, superficial reasons to get into acting. There are so many people who display poor acting skills over this. Yes, your drive should want you to be rewarded someday or noticed– nothing wrong with that– but it must be for the right reasons or your work will be crappy.
Study real actors and performances, study the best. Think about them deeply for months, for years if needed, until you know the answers to all your questions. Is it possible for actors to know how to play any role? Yes, but you have to be a deep thinker. Consider all the circumstances, know yourself to a T and apply serious consideration and every step in developing your character. The portrayal is a combination of facets– the depths of your spirit, the physicality of the character from head to toe. You have to have considered and filled in every blank about the role.
Now, getting to the point as an actor of being able to play any kind of person or motif takes time but there are actors out there that can really do it because it's part God-given talent and serious work ethic/drive. If you can afford to study, do so and with a reputable school or experienced coach with real credits. Acting is much more involved than what it looks, respect it. Harness yourself, really find out what this is... what the sensation of real acting feels like. Don't avoid it and fear it, if you really dream of being great... be brave and step into the fire of discovery and come out like the phoenix. Be ready for inner battle in learning and discovering your craft, it's a jungle in there.