How to start preparing for iitjee 2018 from class11 and most inmportantly how to prepare myself mentally to get ready to fight for the upcoming challenges and eak points.

Please follow the given links.

Just read them in general not for JEE 2015 or 16.
http://www.askiitians.com/iit-jee/main/preparation-tips/
http://www.askiitians.com/iit-jee/how-to-prepare-for-iit-jee.html
http://www.askiitians.com/iit-jee/advanced/preparation-tips.html