My advice would be to first think about what area of archaeology you are interested in, as each university has it's own particular specialisms. What these specialisms are depends on the specialisms of the lecturers and researchers at that university - all will be able to teach the introductory courses, but at higher levels they will teach what they know, e.g. the Prehistoric Near East, the European Mesolithic, Heritage Management, Death and Burial, or whatever. Most universities will have several specialisms and you should be able to get an idea from looking at the course modules available to choose from.
Do you want to do a BA or a BSC? A BA has some practical elements, and you will go on digs (this is an archaeology degree requirement at almost every uni) but it's focus is more theoretical, whereas as BSC is more science based, in the lab sort of thing, but you will still also dig.
Being firm in these decisions will help you get ahead as a lot of first years arrive at uni without really knowing what they want to study particularly; if you have an idea from the off (even if this changes or evolves) you'll be more clued up.
Chris Scarre's The Human Past is a very good textbook, as the previous commenter said. It's often recommended for first years and it gives you a great overview of world history and archaeology - and it's really interesting. You can get it from Amazon for less than a tenner.
Work experience and volunteering wise - see if you have a local archaeology society; any digs they do will be powered by volunteers, so they usually welcome newcomers, but also don't expect you to have any prior training, or your own equipment (they'll provide it; just turn up with a decent set of waterproofs, a hat, and some sturdy shoes, preferrably with kelvar or steel toe caps). Coming up to summer is prime digging time so there'll be plenty of opportunities.
You can also email the uni you're interesting in going to - as they will have summer student digs - and ask about digging opportunities either with them or that they know of.
In terms of finding one independently, the BAJR website is the best one. They also have a Facebook group - it's updated daily with alsorts of information and opportunities so I would recommend joining it. You can also ask them on that page about digs local to you - not all of them are put up on the website, you may get some unofficial opportunities.
Archaeologists often work very closely with museums as that's where the finds end up, at the end of the research project, and also sometimes before that, for conservation, so contacting your local museum and asking if you can volunteer with them is another option you can explore.
I'm doing a PhD in Archaeology (having done a BA (Hons) in Archaeology and Ancient History, and an MA in Archaeology), so if you have any more questions or want any help, send me a message :-)