[header] How to convince your parents to let you buy a nice diary [title] Work on proving your maturity and responsibility. [step] If you want any expensive item, including a diary, you want you show your parents you're mature. Your parents are more likely to trust you with nice things if you've proven your'e responsible. 
For a few weeks before asking your parents, work on showcasing your maturity. [substeps] Keep up with things like schoolwork and household chores.