Hey, I wanted to ask if and how I can, for example, save hunger and thirst in the Steam Cloud.
You can write information into the steamCloud via ISteamRemoteStorage -here is an API reference:
https://partner.steamgames.com/doc/api/ISteamRemoteStorage
An easier way is to use the auto-cloud.
So for user defined settings like savegames (Which should include hunger and thirst) you can simply write to the directories defined by the steam auto-cloud.
https://partner.steamgames.com/doc/features/cloud#steam_auto-cloud
In the end you should make a concept about which informations you want to store where. save games and their respective variables should be stored locally so the steam auto-cloud will take care of themwithout any programming. Then you concentrate on creating the save game files and steam does the rest. If you want ot share some information like highscores or other “interesting” information to the public then you should look into the steam coud api.
@Fronzelneekburm Oki Thanks for the reply. But can the user change this values?
I want make an Multiplayer Game with saving Hunger Thirst, maybe into the Steam Cloud or in a MySQL Database. Which would you to prefer? And did you know if I can talk to my MySQL Database when I use the Varest Plugin? So i can also insert and Update Data in the Table?
sry for bad english 
Normally the user should not be able to change these values -depending on how it is implemented in the game, but since it is a locally generated value you can not prevent that completely (cheating, hacking!).
Personally i would prefer a mySQL database since that will allow you very flexible custom solutions like your own custom SQL statements and you’d be less dependant from steam. mySQL is pretty secure if you follow best practices with prepared statements to prevent hacks like SQL injection. I also like that when you store as much as possible in mySQL you can make pretty awesome statistics - that is very helpful when you have thousands of players and want to get information about what they do often and what they suffer from (for example where most of them fail in the game). That info helps a lot to balance and improve the game if you use it wisely.
The Varest plugin is able to easily create REST server communication. If you want to use REST exclusively I’d make a simple REST api on the webserver of your mySQL database as a frontend. That way you also have the option to implement additional features like security, simple SQL statements for statics purposes (for example - number of users, number of users who are close to starving, etc…).