how can I applyfor btech in navel academy scored 88% in plus two

You must have to go to there official site and filling up of personal particular in the e- application , detailes  to be filled up as a given in matriculation.

Certificates/ 12 markesheet ,field like email address, mobile number is mandatory information. All the revelent documents (preferbly in  original) Date of birth proof (as per 10th/12th certificates), 10th mark sheet, 12th mark sheet, JEE (Main) 2018 scorecard (indicating All India Rank) and a recent size colour photograph should be scanned and uploaded in PDF form. If any scanned document is not legible/ readable for any reason, the application will be rejected.