need seat/shoe advice

Shoes- as a beginner, you may want to wait a little and get used to the bike and all the stuff- braking, shifting, traffic- you have to do at once before you go clipless. It's a whole extra thing to learn, and taking it in steps is prudent. Give it a couple months. Use the stiffest sneakers you have, Tuck the laces into the shoe so they don't catch in the chain. If you demand better power transfer, try clips and straps, easier for beginners. If some guy in a shop is trying to push clipless on you this early, he's in it for the buck, not your best interest. If you just think the shoes are cool and want them- well I think we can all identify with that, just make sure the fit is good. They're all so different. Lots of pointy toes that pinch. You'll like mountain bike shoes better, you can walk in them and use them with out clipless pedals at first. The stiffer sole will give you better power transfer.

Saddle- If the one you have is killing you, welcome to our tribe! You can get a narrower firmer one very reasonably, you may want to try out a $30 or so saddle before you dive into $100 racing saddles. Personaly, I like the hole in the nose type.
It sounds like the seat isn't under your sit bones, and you're riding on the pubic bone (achy!) and the tail bone is getting strained from the bad position. Even achier! Since I haven't seen the saddle or you riding- I could be totally wrong. Sometimes an adjustment will fix it, sometimes you need to switch seats. Take it back to the shop- they should be doing free adjustments for the first few months, and you should be due for the 30 day tune up you get for free with the bike. Cables stretch and stuff on a new bike. Tell them the seat hurts, and if the guy doesn't know what he's doing get another salesman.
Bike shorts help too, and you can get your size from Team Estrogen. I have the Terry Knickers in, either 1X or XXL, love them! Frightenly expensive, but cruise the whole womens section. If you persevere, you won't be 2X that much longer.

Lizzy