I want to do IIT. now I am in 12th. what can I do for it?

For admission in B.Tech in IITs, you need to qualify Jee Main first. Then you have to appear for Jee Advanced. A valid score in Jee Advanced, can help you to get a seat in Top IITs of the country. Now you are in class 12, and this is the right time to start your preparation for Jee Main.
First, you should make a proper and programmatic study schedule which you'll follow throughout yiur preparation. Then go through the entire syllabus. Instead of choosing a random topic from the syllabus, you must break your preparation into sections. Sub-topics from each section should be selected and then strategy on clearing concepts must be made. Make your own short notes or flashcards. Never leave any concepts doubtful or unsolved. Once done with the syllabus and concept clarity, you should move towards solving previous years papers. Give mock tests as much as possible. Time management is one more important thing you should look upon. Remember that practise will increase your efficiency.
NCERT Textbooks of class XI and XII are best for basic preparation. Once done with these books, you can move on to the next.
http://engineering.careers360.com/articles/best-books-for-jee-main/amp