Hello aspirant,

Don't worry and most important never loose confidence. Since, you mentioned that you scored less marks in 11th standard I would highly suggest you to focus on the topics in which you are weak in your summer break because it will have direct impact on your 12th standard syllabus.
Follow these tips to improve your marks-

1. Make a timetable and live upto it.
   
2. Be crystal clear with concepts and practice a lot.
   
3. Stay motivated.
   

Tips to score a high percentage in board exams-https://www.google.com/amp/s/engineering.careers360.com/articles/how-prepare-for-jee-main-and-class-12th-board-exams/amp