You need to dig out the sod and backfill the area with Gravel (I use c-6 or road mix) that packs very well. Make sure it's level and pack it down well. Best way is to rent a packer to get a good firm base. Then form it up and pour.
That is a very, very brief description. If you've never done it before it may pay you to find someone to help you that has experience pouring concrete. Once you start pouring you have a small window of time to get it screeded, troweled flat and finished. If all the preparations up to the point of actually pouring aren't right, you'll have a mess on your hands.
Hope this helps.