My son has used mathamphetamine. What is the best way to deal with
this kind of problems?

Yeah that or they made a typo and meant methamphetamine.  Since they
are paying you to answer the question, one would think you'd be more
forgiving.  Since it's their son and this is a very serious
issue...one would think you'd be more compassionate as well.

limsusa:  I'm certainly not an expert here, but I'd do a Google search
for methamphetamine treatment.  There are a number of hits for this. 
I imagine there is no best way to deal with methamphetamine abuse or
drug abuse in general because it is very situation specific.

How old is your son.

If he's physically an adult "has used" is not likely to be a problem.

"Is using" is more of an issue. Feldersoft is right in saying it's
situation specific. It's a matter of how often, how regularly, and
why? Does he mix it with alcohol? Does he drive when still affected by
it? Does he buy it from a reliable source?

Whatever you do, don't respond to a knee-jerk reaction of the kind
that automatically bans all sex, drugs and rock'n'roll.

My son just got arrested for processing small amount of
metaamphetamines. I do not know what is the best way to deal with this
situration. Should I bail him out or let the court decide what to do
with him? I knew something must be done so that he might be able to
stop using this dangerous drug.

Show him these two photos and tell him that this can happen to him in
only a few months just like it did to her:

http://www.estatevaults.com/bol/images/%20%20before%20and%20after.jpg

tutuzdad-ga

If, by "processing," you mean that your son was making meth on the
premises, please see to it that any chemicals that he may have used
are removed from your home. Your local fire department may be able to
advise you further on this. Meth labs, even small ones, can be very
hazardous.

Thank for the advise. What I mean is possessing not "processing".
Sorry to make the mistake.

"My son just got arrested for [possessing] small amount of
metaamphetamines. I do not know what is the best way to deal with this
situration. Should I bail him out or let the court decide what to do
with him?"
Is he still being held, pending possible bail?  Is he a minor?  In what state?
Some states have good programs for under-aged drug offenders that put
fear of the substance(s) and respect for the law in them  - better
than parental influence.
Talk to a legal counselor for juvenile offenses.
Answers to the questions could help a researcher respond.

My son is 36 years old. He was arrested last Thursday pending for bail
and was transfered to Los Angeles jail on Monday morning. We have no
experience to deal with this type of situation and do not know whether
it is better for him to stay in jail or bail him out? What is the
approach for this type of problems. Is Los Angeles Jail will do
anything to help people with this type of problems>

If I was in your position, I would certainly bail my son out...jail is
not a good place, and does not really teach any lesson. I know
regardless of the circumstances I would bail any of my kids out of
jail. Secondly, I would make conditions for your son to meet after he
is out, such as living with you...other stuff that would put control
on him. I know...having known many people who have ruined their lives
with drugs, and one who OD'd... The only thing i've ever seen work is
to put them in treatment. If he does not want to go, however, there is
not much you can do. Just make sure you stress that you are behind him
fully and love him, and hopefully he will make the right decision.