Stamina is straightforward. You get so much per game day, and you use some up when you perform certain actions (travelling, hunting for monsters, fighting, a few other things). You can see how much stamina you have left in the Head-up Display on the right hand side. How much you use varies according to what you are doing, what skill level you have achieved in this activity, whether your backpack is overloaded and various other things. In order to find out how much stamina it will take to do something, you can click on the link stamina labeling the stamina bar. This will open a new page with a list of all possible activities and how much stamina they will cost you. Note that 1% stamina is 10,000 of whatever units is used there.
The amount of stamina that you get at a new day depends on the state of the Kitty Bonus (bottom of the heads-up) and also your eating habits as detailed below.
NEWS FLASH: No matter what the New Day message says, you WILL NOT STARVE TO DEATH. The worst that ever happens is you wake up the next day with a base of 85% stamina instead of 100%. This does NO lasting damage to your character. If you cannot afford both food and equipment, choose equipment.
Eating food gives you stamina, as you might expect. Anything you eat has a "Daily Energy Requirements Score." This is how much stamina you gain from eating it. For food available in restaurants this score is displayed in the restaurant for your convenience. For ration packs and energy drinks (eg. bought in eBoys) the stamina gains are 10% and 2.5% respectively.
But you can only eat so much in a day. In the heads-up display there should be a bar labeled Hunger. It will very probably be yellow meaning that you're pretty empty. Quite hungry in fact. When you eat you fill up, and your hunger bar empties. If you continue to eat after you are full then your bar will turn from Hunger to Nausea, its colour will turn from yellow to red, and you run the risk of throwing up violently - costing you a lot of that stamina, and making everything else take more stamina for the day. You can work up an appetite by using up stamina - you can eat a bit more in a little while after some work, up to a point.
The stat that measures how much you can eat is mass, and unlike stamina, it has no natural game given units. I use a bar length. That is the length of both the stamina bar in the heads-up display, and also the info bar displayed in the restaurants. Note: If you haven't eaten for a while, your hunger can extend "past the end of the bar" - that is, you can eat a an amount of food before it becomes visible on your hunger bar. In fact if you are very hungry, you can eat up to as much again. Your hunger will return at the rate of one bar a day, if you work your stamina down into the orange each day.
The next stat to know about is the Fat Content. Eat too much fat and you'll get, well, fat. This means that you'll start getting less stamina per day to play with. There is no indication of how fat you are in the heads-up, instead you have to read the New Day screen, where it will tell you that you are something ranging from "You are looking trim and slender! You gain some Stamina!" to "You look like you just ate a schoolbus full of well-fed orphans. You lose some Stamina." If you start getting messages that look more like the latter than the former, then it's time to start looking at low fat foods. If you don't eat anything at all then after a few days the fat will start to burn off by itself. This will happen faster the more you drive you stamina into the orange or even red.
The last stat is nutritional content. This works in the opposite way to fat. High levels of nutritional value is a good thing. As with fat there is no nutrition bar in the heads-up display to keep an eye on, instead you have to rely on new day messages. these range from "You are slowly dying of malnutrition. You lose some Stamina." to "You are feeling strong and energetic! You gain some Stamina!"
That's a lot to get used to. Are there any quick and easy rules of thumb? Well, the best strategy depends on what you are trying to do. If you want to have one gameday of really high stamina - for example, for doing lots of materials-gathering or building and only paying one cig for the kit - then you need to:
1 - not eat for a few gamedays beforehand, to get really hungry,
2 - chose food which has the highest Stamina per Mass ratio. (eg 'The Unrepentant', in PV)
With luck, you may be able to get over 600% stamina in one gameday, not counting any bonuses for superchronospheres, equipment etc.
If on the other hand you want to be generally healthy, you need to eat until no longer hungry every gameday, but not stuff or starve yourself too much. (Robots cannot eat, and Jokers do not get Fat or Malnourished. So this mostly applies to Humans, Kittymorphs, Squats, Mutants and Zombies.) This should give you the best average stamina.
The third strategy, which may be the best when starting out, is to ignore food. As said above, if you're struggling to scrape together the req to buy a meat fork and some embarrassing jim-jams, you don't need to worry too much about eating - your character will not starve to death. It will get all boney, and possibly hate you, but never mind that.
For a detailed explanation and huuuuge list of which carcasses are worth cleaning, go here.