O.K. so I Built my new computer and installed Hardline on it and attempted to start play. However, aside from trying to memorize the new control set up . I have NEVER played ANY of the Battlefield games and have no idea why I can't open the 1st door where I need to use defibrillator (I know the use key is E). It is very frustrating. Is it because match won't start until 4 players on? Help please

are you talking about multiplayer or single player, in multiplayer all doors that can be opened should open even without the 4 players in, in single player i have no idea as i haven't tried it yet

I will figure it out

Go into your loadout - Operator class and make sure you have it added to your class, both sides.  3 or 4 will bring it up depending on what you have setup.  Is this what you are looking for?

Any door that can be opened will have a white circle show as you get close enough, then you get a message "press E"