How do I encourage myself and make myself thankful for what I have?

I admit I take much for granted, I complain because one thing goes wrong or be sad because I don't have the newest iPhone. To me it feels bad being the middle/lower class student in the rich kid high school. However I try very hard to tell myself I should be more grateful and some people don't have anything, I have a warm bed and roof over my head. But yet I always seem to fall back into a state of wanting. How can I change, so I can increase my happiness?

That's a very interesting question. But every question deserves an answer. At least you do have a phone to communicate, so you can be glad about that. If you want to make yourself happy, the Bible has an interesting answer to that question.
Acts 20:35 says,"There's more happiness in giving than there's in receiving."
So instead of focusing on what you don't have, perhaps you can spend your extra time on doing something for others. Maybe an elderly person in your family or community. The more time you spend doing something positive for someone else, the less time you'll have wondering how to be happy, because you'll be busy doing something constructive with your time, which will make you very happy and contented.

Don't compare yourself to others and choose to be grateful for what you have. Things don't bring us true happiness even though marketing makes it seem that way.

I was like that too. Honestly, I helped myself be grateful by watching videos on YouTube of how people with nothing live. Suddenly I wasn't so greedy and I was super thankful for what I had. I mean, I don't know if it will help, it will make you sad for sure.