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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.