JEE

If you are getting a good college, then you might consider computer science branch, but only if you have interest in it. Otherwise, you may get bored.
Even mechanical and civil branch is core branch and you should consider it.
Well, it totally depends on your choice.

To get an accurate answer, you should mention your category. If you are a boy in general category, then you might not get any good government college. If you are a girl, then you would get a good state government college like, College Of Technology, Patnagar.

You can use the book A.D.Gupta. It is the most valued book to practice any topic on mathematics. I would sincerely suggest you to practice according to that book.

If you do not want to drop, then you have to join the college you are getting. It seems, you have cut your choice yourself, still it would be better if you think again.

To wisely prepare for your JEE 2018, you can join any good coaching institute. They will give you proper guidance, and then you can ask your senior for dropper batch modules which contain most of the probable topics for your exam.

That score is good enough to get admission in any good state government engineering college for instance, College Of Technology, Pantnagar and many more.

You should have specified your category, but considering you to be in general category your JEE score would not land you any NIT or any other good engineering government college.

Yes, you will get admission in a government college with such a rank, but the college you will get would not be very good. It will lack in various aspects as compared to various other NITS or IITs.

NIT will not be possible with this score still you can try your luck with home state NIT or some low rank NITs. Else, you can go for some GFTIs or regional engineering colleges and you might get a seat in one of those.