one's sea legs
1. Literally, the ability to walk steadily on a moving ship. Don't worry, once you get your sea legs, you'll be able to walk around the deck no problem.
2. By extension, one's comfort level or familiarity with something. Of course you're scared, it's your first day of teaching! But you'll get your sea legs soon enough, don't worry.
*one's sea legs
Fig. one's ability to tolerate the movement of a ship at sea. (*Typically: get ~; have ~.) Jean was a little awkward on the cruise at first, but in a few days she got her sea legs and was fine. You may feel a little sick until you get your sea legs.