It's actually not that simple. To contact an angel you must not have too much negativity in you. You may contact him/her for help with a problem that is causing the anger but you shouldn't contact them when you are filled with hate.
You must also attract the angel. To attract an angel you must think like one. Help people out, Forgive others, even if it is just helping your parents do yard work you actions will count. If you see a kid in your school drop his stuff, help him pick it up. Once your angel is attracted to you he/she will send you signs and they will contact you more often.
To contact one at this moment is rather complicated. I would recommend this meditation:
though I am not too much of a new age follower this helped me out alot.