Press the green flag twice. Then, select the animal that you want to play as (keep in mind that the animals with the highest stats aren't always to most enjoyable).The animal will follow you mouse pointer, and will accelerate (draining your energy) when you hold down your mouse. This will also increase your damage because F=ma not F=power stat (the power stat does not affect impact damage, the hp and speed do). Press 1, 2, or 3 to switch between different attacks (you will need to press a key once you select an animal as there is no default attack), and press space to attack. Keep in mind that this game is about survival and having fun, not killing everything on sight (many things can one-shot you if you are not careful). Have fun!
Keep in mind that this is early access, and new changes will be made frequently. Be sure to report any bugs!