Which trees drop apples?

Apples are most commonly dropped by Oak Trees. (Why do oak trees drop apples? Because it's Minecraft.) They're also dropped by Dark Oak Trees, but as these only grow in the Roofed Forest biome, you're far more likely to find regular oaks first.

Apples aren't a very good food source though: they're unreliable, and even a lot of them don't keep you fed for long. Easier and more filling than apples is to simply perpetrate some violence on chickens, pigs, sheep, cows, or rabbits and eat the meat raw or cooked in a furnace. In the first day of Minecraft, meat is the simplest, most abundant, and most filling food source.

For vegetarian playthroughs of Minecraft, the easiest early food is bread made from planted wheat: obtain seeds by breaking tall grass, use a hoe to till dirt into farmland beside water, and plant the seeds. (See the farming tutorial for more.) You'll have to survive several hungry days before your wheat is ready to harvest though.

If you're lucky enough to start near a village, the best early vegetarian food source is harvesting and replanting the villagers' fields for your own benefit.