Thank you for your query
There are a lot of reasons to have general anxiety
disorder one of the most important reasons being stress
.also try to observe if one of your family members has it in order see if its genetic.
You haven't mentioned about since when you have been having this feeling.
Use of drug is also another important reason to GAD.
There are a lot of treatments and do not panic .GAD can be very easily treated if you take the right treatment at the right time.
CBT is one the therapies psychologist
use where in your thoughts and feeling will be understood by the therapist, if that has any major affect on your behavior and try to help you out in removing those negative thoughts if any.
There are other antidepressants like SSRI's which will be prescribed by a psychiatrists which you should take if necessary.
Try to involve in healthy habits and activities which you think will help you to relax. There are lot of relaxation techniques
like music, sports etc which will help you to release your tension.
Try to lessen your burden by taking in less tension which could be from anywhere like your school friends and family. Do spend sometime to yourself.
is an easy practice to relax.
Hope this helps.