Answer: The best way to learn is by doing. Look for school plays and community theatres to audition for. Or if that's not available you could do short movies with your friends.
Here are some short scripts you can use to make short movies: http://www.freedrama.net/short.html Then you'll have the short movie to watch and get ideas on how to improve your acting.
You can practice on your own as well with monologues: http://www.freedrama.net/small1.html
After you practice and memorize the monologue, you can act it for friends or family or video yourself doing it and watch it on your own for ideas to improve. If a part is boring or hard to watch, then think of ways to make that part more interesting. Do the monologue over until you feel like you'd be proud to show someone else. When you do show your video monologue to someone else (or even a small group of family and friends), watch how they react rather than watching the video with them. Are there parts that they have good reactions to? Make a note of the good parts and then think of ways to improve the other parts.
Here are some more acting tips as well in the Freedrama.net Free Acting School: http://www.freedrama.net/acting.html