JEE Main

For OBC category, 180+ would be safe for CS branch. Also, it depends on your state, whether state quota or all India quota. But, top NITs are NIT Warangal, Surathkal, Srinaga and Durgapur.
All the best.

There will be no effect on your placement but at least you will do much hard work to get next year and achieve better rank than this year.

First, complete the NCERT book then study the coaching material. Test series comes in last.

If your general rank is 6213, then u'll easily get IT at IIIT Allahabad because of reservation quota.

Nowadays coaching is required for the preparation of all exams. But if you do not want to join a coaching then you should focus on what is taught in school and attend the classes regularly.
The school will prepare you for the boards. for JEE Mains you should thoroughly prepare your school syllabus of class 11&12 as the syllabus for JEE is same. You should solve previous year question papers of JEE and can also buy some books for JEE preparation. Your basics of 11th and 12th class should be clear.
All the best.

Sorry to say but you are not eligible for any of the government colleges including NITs, GFTIs based on your marks.

Sorry to say but maximum of three times a candidate can appear in JEE Mains but for JEE Advanced, this is restricted to just 2 chances. In short you can't appear in JEE Advanced in 2017.

Yes, you are eligible. You need to visit the campus.

Approximately 250 to 290 marks.