You might find these idioms helpful when describing yourself. I teach them in my conversation/idiom classes when we discuss personality. Let me know if you have trouble understanding any of them and I can explain more.
**Down to earth - someone who is humble, a realistic and practical person who has sensible reactions and expectations.
**A loose cannon - someone who cannot be trusted because they are unpredictable and have irresponsible behaviour which can cause trouble.
**As stubborn as a mule - someone who is unwilling to listen to reason or change their mind about something.
**Hard as nails - someone who is very unsympathetic who does not seem to care about others.
**High and mighty - someone who thinks they are more superior than other people.
**Full of hot air - a person who is full of nonsense and talks a lot without saying anything worthwhile.
**A heart of gold - someone who is very kind and caring.
**A pain in the neck - someone who is very annoying.
**A couch potato - someone who is lazy and spends too much time in front of the TV and eating junk food.
**Butter wouldn't melt in his/her mouth - someone who looks completely innocent but they are capable of doing naughty/unpleasant things.
Good luck! I hope this helps! :)