If you are looking at having the system monitored, pretty much any alarm company can do this for you. You could call ADT or you call a local company who's level of service you feel more comfortable with.
As far as using the system, first you need the master code which you should be able to get from the previous owner. If you have a model number of the panel you can search for a users manual on the internet.
Typically ADT uses one of two panels, one of them you arm and disarm by entering the code the other you need the code followed by <arm> or <off>.
Changing codes will be very panel specific and you need to know the panel make to find out how to do that.
http://www.keepsafesystems.ca - KeepSafe Systems installs services and provides alarm monitoring services for security alarm systems in the Vancouver area of the Lower Mainland Fraser Valley British Columbia.
The KeepSafe Systems service area includes but is not limited to North Vancouver, West Vancouver, Richmond, Delta, Tsawassen, Ladner, Surrey, Cloverdale, Abbotsford, Mission, Aldergrove, Maple Ridge, Haney, Port Coquitlam, Coquitlam, Port Moody, Burnaby, New Westminster, Kamloops, Kelowna, Courtney, Comox, Nanaimo, Lions Bay, Squamish and pretty much everywhere in between.