1. Find a couple of actors you like. Emulate them. Delve into them. What are they doing to get into character? What are they doing to stay in character? How will you be able to do that as well? These are questions that once you've answered, you can start applying to virtually every role. Simply being able to do things on command is not enough. Just because the emotion is right does not mean that the character has been made real. You have to become your character, blur the line between actor and character to the point where it's hard to tell which is which.
2. Find a scene that has several vastly different emotions throughout. Read it, understand it, and look even deeper. Try to find the deepest motivations for your character. Why are they Emotion A here? What causes them to become Emotion B? And then why are they now Emotion C? Often times, these answers are not obvious. Often times, you'll need to invent a reason yourself, but you have to make it believable. It has to fall under the conditions already laid out by the scene and maybe even previous scenes.
3. Watch "Whose Line is it Anyway?" religiously. Seriously. Yeah, it's improv, but they way in which the cast is able to quickly create entirely believable characters is an amazing skill to have. Sometimes it helps to base a character off of someone you know, maybe even a mix of people. By grounding characters in traits observed from reality, it adds to the credibility.
4. Observe. Observe everyone you possibly can. What they do, how they do it. Try to ask yourself why they do what they do. Once you can understand people in such a manner, look at your character. What's a situation this character is in that you've seen before? Make it relate to real life. The whole point of acting is to sell a character that is part of a story. You're telling a story. People are more inclined to believe a story when they can relate to the characters in any way, no matter how small.
If you're looking to get into acting, search around for places that are looking for actors. Take classes, be it in school or elsewhere. This is the best field to exemplify that experience is the greatest teacher of all. Never stop practicing. There's a million ways to practice, even things as simple as doing impressions of people or characters.