I really think it would be great idea to put an English dictionary app on your phone. These days, almost all my students have a smartphone (usually an iPhone) but hardly any of them have an English dictionary app on it.
While many students bring a bilingual Japanese-English dictionary electronic dictionary to class, many students have no dictionary at all. However, I think that using an English dictionary regularly can be one of the best things you can do to improve your English. Indeed, I try to get students in my class to use their dictionaries for at least a few minutes every class. Only by looking up a word they don't know and seeing it in an example sentence can they increase the size of their English vocabulary.
My recommendation is the Cambridge Advanced Learner's Dictionary app. You can buy it from the iTunes store or Google Play depending on whether you want it for an Apple or Android phone. This dictionary is designed for learner's of English and has easy to understand definitions of thousands of English words (about 170,000). Importantly, it also has example sentences showing how a word is really used in a sentence. I have this app on my iPad and it is very easy to use.
I know that one reason students don't buy an English dictionary app for their phone is that these apps are quite expensive (this app costs 1,400 yen at the iTunes store). However, I'm sure that if you explain to your parents what the app is for, they will help you with the cost of buying it.
Just think how useful this will be. You take your phone everywhere with you. If you have the Cambridge Advanced Learner's dictionary app on your phone, then you will always have access to thousands of English words, definitions and example sentences. There will be no need to carry an electronic or paper dictionary to your reading or writing class anymore. Moreover, you will find that you begin to learn more and more English words. You will also become a more independent learner of English.