I'm scared to death I might have either motor neurons disease or MS

I know this is probably stupid asking questions on the internet about problems like this, but I'm a 16 year old boy and physically fit and healthy, however I have been experiencing leg twitches for weeks then last Monday I experience a burning feeling in the right arm then the next day, I felt as if I was losing grip on that arm then by the next day, I lost grip to some degree in both arms, but not majorly and they feel quite weak and tingly and i get pains and needles sort of feelings on my limbs. My legs feel pretty weak when going downstairs and my lower back, shoulders and neck vibrates or feels like it's shaking when I move them. I'm so scared 😭😭😭😭😭

deffo contact the doctor tomorrow. they get paid to help you and you’re better knowing if it’s something serious

That doesn't sounds very good.
I recommend you booking an appointment with your gp and explaining the situation which they can diagnose.
However, if you feel scared and it's hqappening now, I would call 111 and ask for advice