You must start preparing with ncertbooks first and focus on fundamental first. Take studues ahead for advance level cincepts using books ljke OP Tandon which is best recommended for indepth and higher level problems.

Practice as many mocks as you can and mpre tgan 10. Analysis of all repetitive mistakes is must and focus on such problems. 

Keep revision of all such mistakes and take short notes. Be calm and follow strict timetables .