You can either store the information in a format that the user can't easily figure out or encrypt the information.
To save in a format the player can't easily figure out, you would convert the data prior to saving to something like a bit stream. A bit stream is a line of 0s and 1s built from converting your information to bits, zero padding each piece so the stream has each piece of information in exactly the same location, then appending all together so it is one long line. You have to parse and convert this back to the actually data when the file is loaded. This type of save is still able to be edited by more clever users but it will be harder.
Encryption may be easier if you use a plugin. There are couple encryption plugins. I haven't tried them but they may be what you are looking for. You simply run the encrypt action when saving. Then decrypt when loading. This will make the file nearly impossible to modify and if the user tries to edit it, it will most likely corrupt the file. The only way to see and change the file information is with the correct encryption key and algorithm.