I am a teacher from Italy.I teach English as a foreign language in a secondary school.
I've recently read in a book : "I don't find it easy running up the stairs"
I've always used this form: " I don't find it easy to run up the stairs."
which is the right form? Can I use both of them?
Both are correct. However, note that if you move "easy" to the end of the sentence, only one construction works:
I don't find running up the stairs easy.
(You can't say "I don't find to run up the stairs easy".)
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.