Socialization
In Chihuahua, perhaps more than in any other breed, socialization is crucial
to ensuring that your puppy is happy, well-adjusted and well-behaved.  Many
people have cited instances of Chihuahuas being mean, unsocial and
intolerable to strangers and other dogs, and that characterization of this breed
breaks my heart.  Chihuahuas are raised to be, quite literally, man's best
friend.  This breed is not a working breed.  They don't herd or hunt.  They are
bred to be companion animals, and they do attach very strongly to their
primary caregivers, but that does not mean that they need to behave like
living terrors to everyone else.  The key to preventing that is thorough
socialization

Our puppies are raised in our home, where we work from the day they are
born to socialize them.  The pups are handled and loved on every day,
usually several time a day, from the minute they are born until they leave us.   
They are introduced to other breeds whenever possible, new sights, sounds
and textures, and other people.  We do our best to give you a great start on
their socialization from the beginning.

However, this next sentence is the most crucial part of this page.  
Socialization does not stop here.  Because this is so important, I will say it
again.  SOCIALIZATION DOES NOT STOP HERE!  In fact, our efforts to
socialize your puppy are only the beginning of the process.  When your pup is
ready to leave, at about 8 weeks old, he/she is in the very beginning stages of
the critical socialization period, which begins around 8 weeks and ends
around 16 weeks of age.  That means that it will be largely up to you to
complete the socialization process for your puppy.  

During this critical period, it is essential that you introduce your puppy to as
many new things as possible.  You will want to introduce him/her to other
breeds of dog, other types of animals, many different types of sounds, floor
surfaces, scents, and especially other people.  You should begin with a
gradual introduction as your puppy settles in to your home, but once settled
in, you will want to get very serious about the process very quickly.  

The first step is to take your puppy with you everywhere that you possibly
can take him safely.  Let him get used to riding in the car, experiencing new
places, meeting new people and/or other dogs, and sniffing the world around
him in a calm and relaxed way.  If you wish to use specific training sessions
to enhance socialization, generally, short, calm and relaxed sessions are
best.  

For example, if you are introducing him to a new sound, keep it at around 2
minutes at first, and try to introduce the sound during a cuddle or play
session.  If the sound startles him, try using a sound app or website that
allows you to adjust the volume.  Start out softer and gradually get louder
until you can play the sound at about its normal volume without startling your
puppy (this works well for things like vacuum cleaners).  

With proper care and effort, your puppy will be very social with other
people and other dogs, and will be confident and comfortable in new
environments and situations.  I have included some links below that offer
more helpful tips and methods of socialization.  

Links:

Tips from Whole Dog Journal

Quick and Dirty Tips

Introducing to Dogs and Cats

Separation Anxiety

Jumping and Barking

Moving to a New Home