Assuming you are talking about NITs as the cut off varies college, category and year wise and there is nothing out for Jee main 2018 cut off but as per previous year information, to get admission in top NITs and in top branches like CS,ME, ECE etc you will have to get score in range of 230-270 as this would be the safest score.

Also, to check the previous year cut off you can go through the below link-

Jee Main Cut off

Good Luck!!!