Before you are creating your first model in Blockbench, you should decide for which purpose you want to create the model. Depending on which game or platform you are working with, you need to take different steps in order to get a working model.
Unity, Sketchfab, Blender etc.
All the platforms listed above and many more support the .OBJ model format. To further your possibilities, you can disable Restricted Canvas and Restricted Rotations in the settings.
Minecraft: Java Edition
Block or Item Model
Minecraft: Java Edition has a very easy way to change every block and item model. To create it, you can use the default settings of Blockbench. With item models, to change how they look in different positions in the game, you can enter Display mode (top right corner) and change how it is positioned.
If you have any questions about or need help with block and item models, use the chat #help-vanilla-java on the Blockbench Discord server.
There is no way to modify entity models in vanilla Minecraft. But, if you are making the model just for yourself or other people who use OptiFine, you can change the model of some entities using OptiFine and Blockbench. To learn more about OptiFine models, read this guide: https://blockbench.net/2018/12/12/modeling-entities-with-optifine
If you want to add custom entity models to Minecraft via Forge mods, you can do that with .java entity models. As all entity models in Blockbench, they come with some limitations. You also need a bit of modding knowledge to add them to a mod. To create an entity model, go to the menu File > New > New Entity Model. Make sure to enter a mob geometry name.
If you have any questions about or need help with models for Minecraft mods, use the chat #help-modded-java on the Blockbench Discord server.
You cannot change the model of armor in vanilla Minecraft.
Minecraft: Bedrock Edition
Block or Item Model
Minecraft: Bedrock Edition does not support custom block or item models currently. Depending on the situation, you can use an entity model instead.
Entity models are the main and currently only kind of models that Bedrock Edition supports. To create an entity model, go to the menu File > New > New Entity Model. Make sure to enter a mob geometry name.
Entity models have some unique properties that are different from regular models. A) They are build on top of a bone structure. Only bones can have rotations and be parented to one another. B) They use box UV mapping, which means that each cube uses a fixed area on the texture map from which it picks the textures for each side depending on the dimensions of the cube.
If you have any questions about or need help with Bedrock entity models, use the chat #help-bedrock-edition on the Blockbench Discord server.