You should only submit material that you've worked on as well as you can. Don't send first drafts and don't submit material that you haven't looked over with regards to spelling and language. This is for your own good — you'll get better feedback if people see you are making an effort to send in "clean" material.
Let readers know if there is anything in particular that you want them to keep in mind while reading your story.
It can be hard to accept criticism. Don't argue with the critic, but if there is anything in your crit that you don't understand, by all means, ask him about it.
Bear in mind that the crit is only one person's opinion. He isn't necessarily right, but his views are examples of how readers might see and interpret your story. That is useful to you, whether you agree with his opinions or not.
Be critical about what you want to use of the critique you receive. Remember, you are the author and this is your story, your style. You decide, others suggest. Some authors get lost trying to please everyone. Don't let that happen to you
Gradually, people learn to view a critique as helpful suggestions and not as a personal attack — even if the comments are negative. Be prepared for a rough start, just keep in mind that with time you learn to accept criticism, use what you see as valid points, and throw away the rest.