Can anyone tell me about the subclass?
what the subclass for? is it affected to the main class?
And what is the best subclass for tyrant?

please anyone help me

you need 1rst to do the quest
then you can have a second char into your own cha
for example you can change to main and to sub-class whenever you want (only in towns)
your main remains for example 76 and you beging sub-class from 40lvl you lvl it up till 80 thats the limit.
you have benefits for main class some passives mainly skills
there is no best sub-class.If you mean best benefit for your main then you have to decide yourself.
p.s. please someone post sub-class benefits i dont have it atm in that pc

There is one thing I need to know: what Chronicle are u playing?

In older versions of the game the subclass has no effect on the main, its like having a separate char
In the newest versions like Graci, the subclass has some effects, since u can learn a few bonus skills on main according to ur sub's

anyway search more here: http://www.lineage2.com