Buick lasabre wont turn over?

I have a buick lasabre (dont ask me the
year. I cant for the life of me remember
right now) its an older car, late 90's early
2000's. Anyways. Any time i open either
car door i hear a clicking noise and my
headlights and break lights blink for a
solid 5 or 6 minutes and then they stop.
As soon as i start my car my lights work
nornally. (Except i cant turn them off
when the car is running) well tonight i was
in my car listening to the radio, the car
was not on, and my radio cut out. Not
sure of what was wrong i decided to try to
start it and it wont turn over. It makes no
noise, like its not even trying to, but my
radio will start back up (for less than a
minute then it dies again) and my interior
and dashlights all work fine. After this
happened i noticed that my lights stopped
doing the flickering thing and the clicking
stopped.  Im not sure if its related either,
but every once in a while if my car is in
park my RPMs will drop all the way to 0
and i have to turn the car off and restart it
in order to go anywhere. When this
happens it starts right up. How do i fix
these issues? And can anyone give me a
rough estimate of a cost? I know literally
nothing about cars and buying a new car
is 100% out of the question. Any help or
advice is super appreciated.