I'm in my mid-teens.

When I'm walking, sometimes my knees feel like they are about to crack
(you know the feeling when your fingers' joints feel like that? So you
'crack your knuckles'? That feeling.) So in order to relieve myself of
it, I normally just stop walking then I move my leg back and forth
(from the knee-joint, downwards) so that it can crack. What the heck
is causing this? I remember that knuckle-cracking is due to the fact
that there are bubbles there, and 'cracking' your knuckles actually
refers to the fact that you are just releasing the bubbles in the
knuckles' joints? Is this the same idea with my knee? What should I do
to fix it?

I have similar symptoms in one of my knees.  For me, the problem was
diagnosed as a slight meniscus tear.  (If it were a more significant
meniscus tear, or if the symptoms had gotten worse, I might have
sought arthroscopic surgery.  But it's just a minor annoyance in my
case.)  I would suggest that you consult a physician, because you
might have a different problem.

Just to clarify: I have no reason to believe that you do or don't have
a different problem.  I'm not a medical expert.  But since everyone is
different, it's always possible that similar symptoms can be produced
by a different cause.

No idea why yours is doing that.  Can tell you about my right knee
which does the same thing.  I've been to multiple doctors, PT, MRI's,
etc.  None can "find" anything specific enough that surgery would fix.
 In high school soccer I once got kicked in the kneecap & it got
dislocated.  Everyone ASSumes thats what caused the damage.

Some doctors say not to exercise, PT tells you to exercise. 
Personally exercise helps my situation.  There are 2 bands of muscle
on the top of your leg that connect to the kneecap.  PT found that 1
band (left muscle of my right leg) was weaker & they guessed that the
other stronger muscle may be pulling my kneecap.  So for me exercise
to keep those 2 muscles equal definitely helps.

I'm 34 now, have had this for 20 yrs (OMG).  Gets worse when its going
to rain or after helping a friend move or something else that is an
all-day event.  Otherwise all the advise I've gotten, and can give,
boils down to "suck it up"
Sorry & good luck

There are two likely causes for this in someone your age. One is
called "Patello Femoral Syndrome " and the other is "Ilio Tibial Band
Syndrome ". In both of these cases either the patella ( knee Cap )is
riding improperly in the groove of the femur ( Thigh Bone ).
Regardless of the the actual cause, specific targeted exercises will
help you to build up the musculature in the araea that surrounds the
knee joint in such a way that it will help to compensate for this. I
would strongly suggest that you seek out an Orthopedic Surgeon or
Sports Medicine specialist who can 1) Diagnose the cause and 2) give
you specific non surgical treatments.