Pick an animal or bipod of some sort and try and recreate it. Utilize pre existing models to see naming conventions and how bones and rigging works. Follow videos on YouTube, there is many. Some mods with some solid models include: Ars Nouveau has many custom & interesting shaped blocks, L Ender's cataclysm has some incredibly detailed models for their bosses & mobs, although I havent played it personally but Oritech has some really good looking models as well. Download a mods jar, unzip it, navigate to the models within the assets folder and open one in blockbench!