Should I apply for JEE again? I have scored 30 marks in JEE Mains and I am expecting around 85% in 12th.

I advise you to go for a drop, attend coaching and reanalyze your progress. JEE Mains and Advance has tough competition. Also, side by side explore your interests, i.e. do some short courses related to your interested field. If you opt for CS in future do learn C programming along. People are moving

There are a number of private colleges to get in. If you can manage to get into a good private engineering college then I would suggest you to go for it. Taking a year drop is not easy. You really need a lot of hard work and determination for it.

You don't have to worry as you have got low marks in JEE exam. With this score you are not eligible to appear for JEE Advanced. But there are colleges that admit based on both JEE main scores as well as board.