While cleaning a chimney can be done by the home owner, it is much more desirable to have a professional chimney cleaner do the job. The professional can also inspect the chimney for any damage that may need repair before you use the fireplace. This is a very messy job.
You will need to get a chimney steeps broom. This is a collapsable wire brush on a long pole. You will vigorusly brush out the chimney (from the roof) to remove all the cresote. If not you are risking a chimney fire.
I would probably go with hiring someone since they know what they are doing. You want to be careful when cleaning things like this so that you do not get hurt. They are professionals and know what they are doing. I do not know how much it will cost though.