Welcome, Sarah! Congrats on the new ride; I'm sure you'll love it. In terms of avoiding a sore butt, it's kind of inevitable that your sitz bones will ache a little at first as they adjust to the new position. The key is to get a comfortable saddle. Some companies, like Specialized, let you measure your sitz bones for a more accurate saddle fit. A saddle is one component worth splurging on as it can make or break your desire to ride. And make sure you leave the shop with a good bike fit. Many shops will include a fit with your purchase. Happy riding!