What percent or ranking should I get in jee mains for getting admission in best NITs and some other tips for career options in engineering?.

First of there are two types of quotas in NITs , one base on your category and other on your state. The NIT present in your state gives you advantage of home state quota ,suppose you belong to Haryana then 50% seats are reserved for Haryana students in NIT Kurukshetra.

Now depending upon your home state and category closing ranks varies for same choices.
You can consider following NITs as top one
NIT Warangal
NIT Trichy
NIT Allahabad
NIT Calicut
NIT Surathkal
NIT Jaipur

You can now take help of carrer360 rank predictor to calculate rank vs marks and closing ranks of various colleges.

If you want best nit trough the mains your ranking depends on your catagary and also on marks if you want best bit and you belong to general catagary you have minimum 130-145 marks in jee mains and maximum 360 . you get best Nit  in these Mark's who's (nit) placement is 7-8 LPA and above
And best of luck for her mains

A score of minimum 160 plus would suffice to land in top 10 NITs
NIT trichy
NIT warangal
NIT surathkal
NIT silchar
NIT srinagar
NIT ap