Yes, you can start your preparation after completion of your intermediate exam.

There are various exams in which you can apply after 12th , like: SSC CHSL, Stenographer, Clerk, MTS(multitasking).

There are mainly 3 different phases which you have to clear.

Tier 1- Computer based test. In this you have to study general intelligence, general awareness, quantitative aptitude and english language.

Tier 2 - It is the descriptive paperto test written ability of candidates in english/Hindi.

Tier 3 - It is a computer proficiency test which included typing/ skill test.