I'm using time machine to make backups everytime. Now I'm going to bring my laptop to repair center, and there is a chance that they will replace it to me. It means that I could get back totally clean OS. Will "time machine" help me to get everything what I had before ?
Yes, Time Machine should restore all your applications, personal files, and preferences, provided you have not made any files private the Time Machine system preferences pane.
When you get your new Mac (or freshly installed OS), it will ask if you want to restore from a Time Machine backup, and I think the process is self-explanatory from there.
One caveat - if you have partitioned your hard drive, I believe only the first partition will be automatically restored. You'll have to manually restore the others.