Can anyone help? I just installed an instant hot water at my kitchen
sink, and when I went to plug it in, I find that both of the
electrical sockets under the sink are tied to the wall switch. (There
is just one electrical receptacle, with two sockets in it). My
garbarge disposal plugs into one of the sockets and is operated with
this switch.

I need to change one socket to be always hot, and leave the other
outlet tied to the switch, so I can operate the garbage disposal as
desired.

Someone on one of the sites said that I need to "break the metal tab
between the screw terminals on the receptacle". It said this would
"isolate the two single receptacles and I can switch the hot lets for
one of them". I do not know what this means. Do I need to add more
wires? I do not want to spend the money to have an electrician if I
can possibly help it.

I am NOT an electrician, but when I moved into my house about 30 years
ago, the electricain showed me how to do it.  I had to make sure that
the power was off to the outlet (check and double check).

I removed the outlet from the wall, and there was a small piece of
metal (a bridge) on the side (I think only one side - but not sure)
that I had to grab with needle-nose pliers, and break apart.

I think installed the outlet back into the wall.  One outlet was live
all the time (for my clock), and the other was switched (for my
light).

It was very easy.  However, it was about 30 years ago, and I don't
know if the outlets are still made that way, or even if my memory is
exactly right for the preceeding description.

Either jsut try it (and have a spare outlet ready), or stop into a
Home Depot or Lowes, and ask one of their helpers.  They are sometimes
very helpful.

Those breakaway tabs simply connect the two outlets to each other. You
do have to remove them if you want only one side to be switched. It
may or may not be easy to do this, depending on what wires come into
the ourlet box. The only way to know is to pull the socket out (after
turning off the power) and see how many wires enter the box. If it is
only two, you are out of luck. If there are three or more, you will
need to figure out which is which, then attach the hot and neutral
wires to one socket and leave the other socket wired the way it is.
Usually, this will require you to cut at least one wire and splice
into it using wire nuts. Don't do any of this unless you know exactly
what you are doing.