I'm going to buy a cheap phone plan with my money even though my parents said no
What are you going to tell them when they see you with a phone?
It will be so hard for me to explain this to them
If they have told you no, why not listen to them? They probably had good reason to tell you that.
I think I will be able to hide it from them.
It is wrong to do things behind your parents back. You should have some respect they only want the best for you. They could be trying to protect you from social media or creepers online.