jump in with both feet
To begin or undertake something quickly, enthusiastically, and without trepidation. I know you're nervous about starting school, but you just need to jump in with both feet and do your best!
jump in with both feet, to
To enter wholeheartedly. This phrase is actually redundant, since to jump means to leap with the feet together as opposed to hopping on one foot. A twentieth-century Americanism, it may allude to jumping into a pool, rather than testing the water with one foot.