Dear Shiv ,

I hope you are doing well ,

It totally depends upon you ,along with 12th preparation you can start also Preparation for JEE .It is good during 12th you start your JEE preparation .No,basis of JEE main preparation you cant appear all exam because all entrance exam having different pattern with Different subject like , if you are prepare of JEE  then you must prepare physics,Chemistry ,Math and English subjects but in NEET you must prepare Biology ,Physics ,chemistry and English .So accordingly choose any of one from beginning .

May be not required but 12th marks is most important for your carrier ,so dont only focus on entrance test .

Thanks