http://www.armyjobs.mod.uk/TerritorialArmy/forms/taformrequest
As already said, get the DVD.
You are liable to be deployed with the regulars, although they 'usually' take volunteers. However, depending on your specialisation you could be pulled to go, no arguments. Your civilian employment is legally protected whilst you are deployed. This is not some kind of club, once you are in, trained and qualified the Army expect a return, they pay you, so expect you to respond to their demands. The regulars are overstretched big style, so reservists with skills are the obvious pool of resources they turn to.
There are two types of units, depends on which is nearer your work/home, which is often the best one to join. All you need to know is on the TA web site, or contact your local TA centre.
National Units: If you have a skill or trade that the Army is in need of, you may be able to join a National Unit. In some areas, you may also be able to train for a specialist role that is new to you.
National Units provide expertise in areas such as logistics, IT and communications, medicine, engineering and telecommunications. There are 11 National Unit HQs located across the UK.
National Units require you to attend a minimum of 19 days training per year, including some weekends and an annual two-week camp.
Independent Units: By joining an Independent Unit you commit yourself to a minimum of 27 days training per year, including a weekly training evening, some weekends (typically once a month) and an annual two-week camp.