>>1 Is your dream something you've wanted to do for a long time that you will definitely always want to do? What I mean is that if it's more of a whim or that you'll probably want to do something else later then maybe your family can see that and don't want you to follow something unrealistic. I don't know your situation or how strongly you feel about it but just make sure you really want to do what your dream is. The reason I'm saying this is because I've always been purposeless and haven't ever had a real dream so make sure that whatever you go for is real.
If you're absolutely sure about what you want to do then I agree with the other dollars that you should go for it. Parents don't understand us or see what we see so if that happends then you often can't rely on them for help. Like they said we are here to help as much as we can but since we can only give emotional support and advice you will probably have to work through this on your own irl. If no one you know will teach you then you might have to go out of your way to learn by means of books/internet (if it's knowledge you're after) or by contacting people near you that can help point you towards possible means of doing so.
As for you being useless, it's true that you just have to learn. It's unfair for people to call you useless if noone teaches you anything or helps you learn. If you're having trouble learning then it might not be the right thing for you. I think people think of you as useless because you're not interested in/ good at what they want and expected you to be good at. There's nothing you can do about that because it just turns out that what you are interested in is not what your parents wanted you to be interested in. It's not their choice, it's yours. You may seem useless in the eyes of people around you but you will find that other people will think of you as really usefull and important.
Well that's all I got. I hope I helped :D