You'll need to find an agency and register. Normally they just ask for a photo of you and ask you questions as to what you can do i.e dance etc and your measurements. Don't worry if you feel you don't have a lot to offer because the most they're looking for is people to be in the background. Don't worry about measurements either, they look for all sorts of people.
Rate of pay can be good, and it can be really enjoyable work, the only drawback is that you need to be quite flexible. The work tends to be quite few and far between, and when you do get work more often than not it will be at only a few days' notice.
You need to consider: will I be able to get to where ever the location is? and can I get there for any time? I wouldn't rely on this for full-time employment.
If you stick with the one agency then you will be on the first choice list, if you're with a few agencies then you will be put on a different list and won't be considered first. You also need to consider wardrobe flexibility (they'll say don't wear anything with stripes or spots: make you stand out more than the actors, and nothing with logos: like fashion logos such as Nike etc. sometimes they'll tell you what they want you to wear, sometimes the clothes are provided)
Would you be prepared to do scenes in swimming pools or dance etc? If not don't worry. And above all would you be prepared to wait for hours sometimes just to do a scene that would probably only take 5mins if done properly but isnt and is repeated over a number of times? I can be a lot of fun, and with each one that you do the more experienced you become and so then they'll start to give you work with more popular programmes of which you can meet some famous people.
Any more questions I'd be happy to help.