What part of electronics should I study ?

Well I want to know how to do DIY things at home ,what are the basic topics I need to know for example knowing the system of electronics functions of pieces?for wiring what are the data chips installed what are they for ? I really wish to know how to assemble self made electronic things like a self made sensor laser alarm , or know to install cables for cameras things like that how to compensate the needed voltage is there any straight topics to that if not what kind of studying do i need ??