"It is useful to me" is a complete thought and doesn't need any further information However, "It is useful for me" is the beginning of a thought and needs to be completed. Let's say 'it' is your hand. It is useful for me to tie my shoe. It is useful for me to cook. I hope this helped!

It is useful for me (for some purpose, or to do something with) - it is useful for me to go on italki to get help with my studies. It is useful to me means something is of use to me. I like learning english; it is useful to me in my work.

Both are correct. It's better to study these prepositions with clear examples to understand the difference.