I think I'm too nice
Why do you think you are too nice?
I am afraid people will take advantage of me one day.
I think you're a perfectly nice person and don't need to be nice to others just for them to like you!  That being said, being nice to others in general is perfectly fine, even encouraged!
Maybe I need to start being mean so people won't cross me.
Do not start being mean to others. There is no such thing as being too nice to someone. Keep on being the person you are and don't change for others.