Inhaling helium can cause your voice to change over an octave from its normal tone. To inhale helium from a balloon you just need to breathe out, then breathe the helium in. After you have done that, talk! You will be amazed as your voice takes on the tone of a rather giggly mouse. But you may still be left with these questions: where did the squeak come from? How did it do that your voice?
The speed (and pitch for that matter) of the sound varies with the molecular weight of the gas that you breathe in. The molecular weight of dry air at 0°C is about 28.964, while helium is about 4.003 at the same temperature. The speed of the sound in dry air (at 0°C again) is about 331.3 m/s, while in helium the speed of sound is 891.2 m/s. Therefore the frequency of the sound in helium is almost 2.7 times higher than the sound in dry air. This causes the octave change in your voice. For a fuller explanation see the article at howstuffworks.com.
One thing that you should be aware of before trying helium inhalation is the risk involved. Since your body has no use for helium when it is inhaled into your lungs, you are essentially holding your breath for that whole time, so remember to breathe! It doesn't take long for someone to loose consciousness due to asphyxia. On rare occasions, people have actually died as a result of inhaling helium.
The really important thing is that you do not inhale helium from a canister. Helium in a balloon is at roughly the same pressure as the atmosphere, so when you breath in a lungful, it won't try to expand very much to take up more room. Helium from a canister is under high pressure. As it is released into the lower pressure atmosphere, the helium gas will expand rapidly, in accordance with Boyle's Law. A balloon can take this as it is designed to expand1, but your lungs cannot, and will rupture, leaving you seriously ill.
Helium that is used for filling balloons is specially mixed to contain a little oxygen along with the helium. This makes the helium less hazardous, but it makes it by no means entirely safe. You should consider the consequences before inhaling the helium. Though unlikely that anything will happen, it cannot be said that inhaling helium is risk free.