Hello Sarala. Improvement exam shall be held post April period in 2019. There are two chances one in july and then one in february. Improvemnt exams can be taken in succeeding year only after 12th and hence no such problem shall be faced. 

The forms are available 3 months prior to the exam on the official site or try contacting your principal. Students can appear for more than one subject in which they have failed. Also you can't select the centre on your own. Don't worry you'll probably get the nearest one. Good luck