Minecraft wiki entry for Beginner%27s_guide, associated text: You start on a small island in the ocean: Go to the highest point on the island and look around for other land. If you see a reasonably-sized continent with trees and animals, swim there. If you see no large land and no animals in sight, you have an "ocean spawn". For a player's first game, it is entirely reasonable to exit this world and try again with a different seed.
You are standing on sand, with no trees or water nearby. You are in a desert. Again, go to high ground and look around for trees and green grass. If you see some, go there before continuing with this guide. If there's no green in sight, this is another situation where you can reasonably toss the world and try again. Deserts are much easier when encountered from outside, but they are missing important things like dirt, wood, and accessible stone, all of which which you really do need to start off the game.
You are in a dark forest, with thick trees, a thick leaf canopy, and giant mushrooms scattered around. This is a particularly unsafe place for a starting character and a beginning player, because the canopy can create areas dark enough for hostile mobs to spawn in daytime. However, if you move quickly, you can probably make it to more open ground in a reasonable amount of time, and start setting up in a more reasonable biome. Don't leave the dark forest behind completely, though — once you have basic armor and weapons, it's a very resource-rich area. Grab some of that plentiful wood once you reach the edge! The giant mushrooms also offer a plentiful source of food, so don't forget to grab some of those! There is a rare chance for a Woodland Mansion to generate in this biome. They are dangerous, but provide you a lot of good resources, so make a note where it is!
The land around you is greyish-purple, with dusty particles, giant mushrooms, and red "cows": Rejoice, for you have received a miracle: a spawn in a mushroom fields biome, where hostile creatures cannot appear (except for underground dungeons). You do need to go elsewhere to find wood, crop seeds, and some other resources, but otherwise, you've gotten into a golden situation here. Once you have even a bit of wood, you can get unlimited food from the mooshrooms, and you have a large area in which you barely need to worry about your safety even at night, a great place to set up your long-term base.