JEE Main

If you are from general category, I would not suggest you to wait as chances are least, try to get better college with the other examination that you have given. Explore all the possibilities and make best out of it.
All the best.

Yes, you can but you need to have a minimum of 60 percentage in your 10+2 and opted physics, chemistry, maths as main subject in your 12th. Here are a few details about your course:
http://www.cuchd.in/uie/computer-science-and-engg/.

If you aren't of general category then you are qualified other wise you have to look for private college if don't want to drop a year. You have to qualify for one entrance exam in getting the admission. There are many college which admission on basis of 12 % like LPU, UPES, ICFAI and many other private college. Civil engineering is very good branch who has scope in government and private sector. For taking admission in government you must qualify for JEE MAIN exam with high marks. If you think that dropping a year will benefit to you in preparing for JEE main then go for it otherwise choose BITS, SRM, KIIT, UPES etc.
Best of luck.

Yes, you will get a cut-off probably around 100.

Hello,
I hope you are doing well. You can check the list of college including NITs and IIITs, you can get according to your marks and rank here.
Use Shiksha JEE College Predictor:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=cconnect&utm_medium=CR_JEECP&utm_campaign=7489993.

Firstly, I would like to appreciate your mindset for aiming 300 marks in JEE main exam. Yes, you can definitely get such mark but all you have to do is to get all your concept clear and keep on hard working continuously and by doing this you can achieve good marks.

You may have chance of getting seat in NIT please visit www.siksha.com/jeemaincollegepredictor.

According to my knowledge, your seat is good enough to get admission in Sharda University. You can try to get admission as Sharda University is a very good institution and you may get admission. So, you can try to get admission.
Good luck.

Admission is based on the rank obtained in NATA. Interested candidates can apply online with the required information. Minimum Eligibility to Apply is a pass mark of 60% in class 12 with Mandatory Subject : Mathematics
Or
Must have completed diploma with at least 60% marks from recognized institute. Regards.

It is based upon your interest to choose between various universities.
I suggest Presidency. Vignan is also not bad.