How much salary I can earn if I start working now?
Probably not as much as you think, if you are coming out of school/university most companies will need to train you to do the job they need.
How an interview process actually goes?
Your school or university possibly has some extra curricular activities which focus on this. There are also organizations which can help you with this, as you don't state your country I can't point you in any directions.
How much chance there is of me being hired by a good company?
About 0% chance, unless they are well staffed and a manager looks kindly on you most good companies have people beating down their door.
Will this be considered unprofessional:
If I refused job offer after getting it?
No, whatever somebody might say it's not unprofessional to refuse a job offer. It however is unkind to waste somebody's time, just as you would find it annoying if somebody wasted your time.
If yes how much impact it will have on future career?
Almost certainly none.
EDIT: After a discussion in the comments I realized that the best practice would really be to do interviews for a part-time job next to your future studies.
You get the answers to the things I left our, such as organizations who can help you learn interviewing skills, with some simple searches with your favourite search engine.