Duckling, you will be asked to take everything off above your waist and put a hospital gown on ( some places will let you just wear a plain, thin t-shirt but it's best to assume that you'll wear a gown ). Remember to take off any necklaces before you go in.If you have very long thick hair or a long plait, it's better to clip it up out of the way - plaits so long that they trail down your back can show up.
The radiographer will call you into the room, check your name, address and date of birth. They may ask you some other questions about your symptoms or your treatment so far, this is so they can write it on the xray form and give the radiologist more information.
They'll ask you to stand with your chest against the detector/cassette, put the backs of your hands on your hips and push your elbows and shoulders forward, which moves your shoulder blades out of the way of your lung fields, if you can't do this because you have painful shoulders etc, the radiographer may position your arms differently. There'll be a minute where the radiographer makes sure that you're straight enough and adjusts the xray field, then they go behind the control panel and tell you to breathe in and hold your breath, this is only for a tiny fraction of a second - maybe 0.25. If it's a digital room they will see the image straight away, look at it for a few moments and decide if they're happy with it, tell you to relax and how to get your results. If it's not digital then they may ask you to wait outside while they put it through the reader and then come to tell you can leave.
If you have any questions at the time just ask the radiographer. They won't tell you if they can see anything wrong on the film but if they think you need more urgent medical treatment they'll ask one of the radiologists for advice and then advise you what to do.
Any more questions, feel free to ask me.