I work for a security integrator in the North East US. That being said I do not work, or gain anything from any websites of companies listed below.
My role in my company has changed over the five years, but I have been active in almost all areas of operations with my current company. Currently I am the Engineering Supervisor heading up system pricing and design so my knowledge point may differ from guys who work in the field.
First you must realize that many companies run differently. Some work with residential and other only work commercial. Some specialize in certain "fields" of the security alarm profession. The four major "fields" that are in the profession are: Intrusion/Burglar Alarm, Access Control, CCTV (Cameras), and Fire Alarm systems. There are many other system you maybe asked to work on. Depending on the company you may only work on one or two systems types. Other companies may call on you to work any system they provide.
Many of the techs we currently have never worked in the industry before being hired. Many companies, like mine, don't always advertise for entry level techs, but are always willing to interview interested canidates. This is because training someone "in house" can benefit both parties. You get training and teh company gets a tech who thinks like the company. Some experienced techs have a hard time switching companies since the products and philosphies differ. A new to the industry tech can quickly succeed since they have no prior knowledge.
The most successful "green" canidates have had expereince in PCs, networks, construction, or other low voltage systems (Like telephones). Having no experience with tools is usually a hard sell. The best way to get your foot in the door is to do as much learning before hand as possible.
Most techincal schools have EST (Electronic Systems Technician) training. Also since the industry is becoming more IP based look into computer network training.
Here are some organizations that deal with training and certifing those in the security alarm field:
Security Industry Association: http://www.siaonline.org/
National Systems Contractors Association: http://www.nsca.org/
National Training Center: http://www.nationaltrainingcenter.net/main.xml
National Institute for Certification in Engineering Technologies: http://www.nicet.org/
This organization focus more on those in the field already, but can give some insight into the backgrounds of the industry.