I worked for them for two years to this September and have left at the moment for university (could have transferred stores but my uni doesn't let us work, and not that I have the time!).
You will be on a 0-hours contract. This makes it superrrrrrrrrrrrrrrrrrr flexible and any time off that you want, you will likely get (apart from certain banned periods e.g. Christmas). The thing with 0-hours is that your hours vary weekly, and they will call you up for covering shifts. As a newbie, it's highly unlikely you will get more than one 4-hr shift per week, but as you get experience there and prove yourself, you'll find them increasing fairly quickly. Some people do get regular hours, but most don't.
You apply online, by uploading your CV etc, but also do go into the store as the managers tend to pick through the usually massive piles of CVs. If the store doesn't have a hiring notice up in the window, it's unlikely they're going to take many, if any people on. When they are hiring, they take on a lot of people, but older (as in longer-serving) staff tend to get way more hours than newbies, even if you're older than us - I've run the clothing side of the shop when the assistant manager has been out, including people several years older than I.
Also, the niceness of the colleagues can vary so much! So you might be lucky, like me, and have so many lovely people and only one or two that really annoy you, or be the opposite and stuck in a crappy store.
If you have any more questions, go for it! Hopefully I answered them all.