You walk into the store and clock in then grab a radio and an ear piece. Walk out to the selling floor and get "chatted in." Chatting in is when the manager tells you about all the sales that the store is having so you can communicate them to the customer. Then if they have you out on the floor selling you go to the men's or women's side and talk to customers and help them find what they want, make suggestions, and tell them about the different fits. In between customers you will probably be fixing clothes making them look presentable for the people shopping.
Depending on when your shift starts and the day you work, you could be doing a few different things. Sunday mornings (at least for my store) was used for cleaning the store, such as vacuuming, dusting, disinfecting. We also used the mornings to run the money to the bank drop-off. Other than that, you'll be helping customers, being a cashier, cleaning the fitting room/clothes, and adjusting the clothes in the store to look more presentable.
Most shifts last 4 hours with a 15 minute break. Express will provide walkie talkies to communicate with coworkers in the store. If you are stationed up front, you are expected to approach customers and "promo" them by letting them know about any sales going on. If you are at the cash wrap, you will perform cashier duties and unpack new stock and place sensors on the items. Throughout your shift, you may perform stock checks, start fitting rooms, and place online orders.