How do you convert ac to dc? why is it necessary to have this capability?

To change AC to DC, you connect the AC to electronic components
called "rectifiers" or "diodes".  They have interesting behavior:  It's very
easy for current to flow through them in one direction, and very hard
for it to flow in the other direction.  After those, you have current that's
only flowing in one direction.  But it still rises from zero to maximum
and back to zero many times each second.  On a graph it looks like
a train of camel's humps walking by behind a wall.  To turn it into nice
smooth DC that has one constant steady voltage ... like a battery ...
you use inductors (coils) and capacitors after the rectifier.

It's necessary because ...

-- It costs less for the power company to send AC from the generating
station to your house.  Why ?  Because less energy is lost in the lines
when it flows from place to place at higher voltage and lower current. 
So the power companies change the voltage several times between the
power station and your house ... up to high voltage to go long distances,
then down to low voltage to go into your house.   These changes are done
with transformers . . . and transformers only work with AC, not with DC !
(Put AC into a transformer, you get AC out of it.  Put DC into a transformer,
you get smoke out of it.  Once for each transformer.)

BUT ...

-- The majority of things you use electricity for in your house DON'T WORK
with AC, and need DC to operate.

Things that can work with AC:
Light bulbs
Toaster
AC Motors, in clocks
Electric stove
Electric heater
Blow dryer
Coffee pot

Things that need DC to do their job.
Each one changes AC to DC inside it:
Laptop
Computer
Printer
Monitor
Wifi modem & router
Cellphone charger
Radio
TV
TV remote
Stereo
Cordless phone
ANYthing with batteries, that has to be recharged once in a while