While in 12th, you should decide which course you should do after completing it - one that interests you for a career. read all that you can about it, know the job profiles that come with the course, pay scales. Empower yourself with the knowledge and then check the colleges you should join for the course. Apply for admissions and get going.

After 12th, you will still have some months but the admissions generally start just after 12th so it is imperative that you should check your options while in 12th itself.