What's the best way to get a good food engine going in Agricola early on?
There are a number of options, but if you've only played a couple of times then a Fireplace + sheep is the way to go.

Sheep are worth two food each, so a pile of sheep is a valuable commodity. If you can keep them in pastures long enough to breed (i.e. without eating them first), then they're a valuable investment, providing you with two action-free food every harvest. Two key points to remember about eating animals:


You need a Fireplace or Cooking Hearth (you can't eat raw animals!)
You may take all animals on a space and cook them immediately, even if you don't have space to keep them on your farm.


The second point is crucial. If your opponent is able to pick up a Fireplace, then you must always be alert to the possibility that he could take all of a given animal, and just convert the lot into food. Of course, you can do the same.

Once you've played a few more games, you'll realise that setting up your food engine correctly every time is much more subtle than this. There are several key food sources, and how you put them together depends to some extent on your Occupations/Minor Improvements, and on what your opponent(s) go for. You can


Eat animals
Bake bread
Eat vegetables
Pick up food from Fishing or other food accumulator spaces
Buy one or more specialist Major Improvements (Basketmaker's Workshop/Joinery/Pottery), and eat resources
Use food-based Occupations or Minor Improvements (if you have them)


Your overall strategy for the game, coupled with how your opponents play, will drive your choice of food engine. There are far too many permutations to discuss them in detail here, but for example if you have Occupations/Minors that aid with plowing, sowing or baking, then bread could be a solid food source. If you can get wood quickly, or if sheep come out early, then animals might be preferred. If you can get to a Cooking Hearth, or if you have a Schnapps Distiller, or a way of getting vegetables very early, then vegetables can be a great food solution. On the other hand, if your opponents take the Ovens before you, then baking may become too inefficient, and you'll have to switch to something else. It all depends.