In order to be a good writer, you have to be attentive. Read books, watch film and television, go to plays, and listen to conversations with your body and mind.
Language elicits a physiological response and the ways in which characters communicate are a guide to each personality.
Film is more about imagery than language and plays are more about language than imagery. There are exceptions to these rules, but to begin writing, it's best to keep it simple. Understand the basics of writing and leave experiments for your next writing project.
How do you create a believable character?
Listen for rhythm in conversations, syntax and vocabulary.
Some writers can write dialogue first to get to the character -- a conversation reveals the character's trajectory. And some writers need to know who the person is first before writing dialogue.
Character building can begin by writing a thorough description of your new people.
Creating a character dossier in which you write 100 details about a person allows for writing believable dialogue. One mistake a lot of new writers make is giving, for example, three characters the same syntax, pauses and vocabulary. In so doing, you use one voice for three people -- the author trapped in his/her head.