"Sit" is probably one of the first commands most owners teach their Chihuahua, right along with 'come', 'stay', and 'down'. It is considered one of the four basic commands every dog should know, even if they never learn another "trick" in their lives. Chihuahuas are very intelligent dogs, but like most intelligent species, they have their own ideas about what they should and shouldn’t do. This can make training them to do things like 'sit' more challenging than it might be with other breeds.
While it might take longer to train your Chihuahua to learn how to sit on command, if you work at making the training fun, he will soon learn to drop into the sitting position as soon as you say "Sit". Even as stubborn as your Chi might be, his desire to please you is always going to win in the end. Be patient and take your time with this training and success is all but guaranteed.
Teaching your pup to sit is one of the first things your dog should be taught. Not only is this a very useful skill for your pup to learn, it is also the stepping stone to many other behavior skills and tricks. Ideally, you want your pup to drop into the sitting position immediately, no matter what he is doing. You can teach him using a vocal command, a hand signal, or a combination of both.
Most people prefer to use vocal commands as they are much easier for both of you to work with. You can, of course, train the two together as this gives you the option of using a hand signal in places where he might not be able to hear you. No matter which training method you decide to use, be sure to stay calm and be patient. This will help keep your pup calm and make it easier for him to learn.
You can teach any age dog to follow the "sit" command, but the younger you start the better. You can start working with your Chihuahua when he is as young as 6 weeks. Once your pup learns to sit, you will be able to use it as a way to distract your pup from many other forms of inappropriate behavior. As for supplies, the only thing you really need is a nice supply of your pup's favorite chewy treats and a quiet place to train.