I m studying 12th science stream at present and I have no idea what to do after 12th....can anyone suggest me?
And if you want to do medical then you have to write competitive examination like NEET, AIIMS

And my favorite subject is computer science , chemistry..
But I'm interested to join in defence
According.to your statement I would suggest B.tech computer science and after engineering degree you should try for NDA for technical side or frontier side .
You should focus on your Physical fitness if you want to go for frontier side in Army and maintain good merit in your graduation.
All the Best
Kindly specify your short introduction , favourite subjects and career goal Accordingly I would suggest you few options.
All the Best