Most people inflate balloons one of two ways. Either by mouth or with an electric inflator. Both methods have advantages and disadvantages.
Inflating balloons by mouth is more fun and gives a person more control of the size of the balloon. However, this method is time consuming, difficult or dangerous for some people, and can spread germs through contact.
Using an electric inflator is faster, cleaner, and easier. However, most people find it to be less fun than inflating them by mouth. Also, this method gives one less control over the size of the balloon.
Another article has some good information for inflating balloons by mouth.