How to improve my English?

I have studied English for many years.I began to study English on 1993, when I was a junior middle school student. I studied well in junior middle school and high middle schoo,and I always can get a good result in exam.when I became a college student,I found it was difficult to study English,the words had so many letters that I can't remember them. bit by bit,I dislike to study english.
After graduation,I seldom used English .I forgot a lot of words and grammar.But I were interested in English,and I wanted to improve my English .I wanted to study american English,and speak american accent exactly and fluently.
I thought of many ways to study,but I never did it completely,I was ashamed to speak English,because I was afraid to make mistakes.
Now,I want to improve my english by listening more,speaking more,writting more,and making conversation with native English speakers.
Thanks to Amy,a american friend.She gives me much helps,though we knew each other just for one day.