Think about what you want and why. Is it an object or an experience? Once you know what it is, ask yourself the following questions. They will prepare you for the conversation ahead: Why do you want this? Why should your parents get it for you? If you can't think of a good answer, then don't ask your parents yet. If you don't know why you want something, they’re less likely to give it to you.
To know what is a "good" reason to your parents, look at what they value. Depending on differences in culture and family this could be many things. Helping with family business and caring for siblings may impress some parents, whereas efforts in school and extracurricular activities may convince other parents more effectively. Figure out when they have praised and appreciated your actions; these are "good" reasons to get you what you want. It's also possible that some parents will be more impressed by logical arguments. Prepare a few good reasons why you want or need this item/trip/experience. It will show your parents you aren’t just acting on impulse or cravings, and have given thought before coming to them. Some "good" examples may be that it will help you progress in school, prepare you for adulthood, or help you grow as a person. Other "good" answers may be that it will feed your imagination, bring you relief from the challenges in your life, or benefit the entire family and/or community. Reasons that focus on self-centered, or irrational wants will be less convincing. For example don’t say, “because my friend has one.” Parents will often take this to mean you are just following fads, want to be like others, and won’t really value the gift. Other not so "good" reasons are that you "just want it," that you “deserve it,” or that you “need to have it.” Also, complaining that they are mean parents if they don't get it for you is also unlikely to work.