what will happen if i go to (for example) spiritualist master and click reset?
and will i become spiritualist, or i ll be martial artist and fighter still?
will i lose any fighter skill?
will i get SP if i lose?
just explain me everything about it please you ll get thank user, promise

Only below level 30 can you use them. They only reset skills. They don't take away master class.
You can use them at level 30 with master class but after that you can only reset skills with green book or brown books.

There is absolutely nothing in game that makes you change class.