Dear,
Nothing is impossible with a little hard work.You can achieve anything if you wish to do it. You can surely do this.Appear the improvement examination to get a good  percentage for 12th and qualify for the JEE.Alongside that prepare for the JEE.
Make your basics clear.
Make a timetable and give each subject equal priority.
Follow books like H.C verma, arihant , pradeep , Cengage to make you accquainted with the questions and help you solve tougher questions.
You can apply for the online test series by different coaching institution to make you accqainted with the exam pattern.

I hope this helps . If you need questions to practice for the JEE you can refer to the following link:
https://engineering.careers360.com/download/sample-papers

https://engineering.careers360.com/download/jee-advanced-sample-papers-previous-years