Yes you can bt try to clear it  in the first year because the dropout year is very difficult to handle. A lots of tension and pressure is there. Your enthusiasm will reduce . So try to clear it in the first attempt.

Yes You can appear.

JEE  Main will be held two times in a year one in January and other in April. The eligibility Criterion include you either you must be appearing in 12th this year or you have already passed 12th.

You can start preparing for JEE Main from NCERT anf gradually you can increse your level. Afyter qualifying JEE Main you can appear for