The first writings were little images, that were supposed to show what
was describes. Later on, people replaced those images by signs that
symbolized a sound. The human voice is capable of forming about 35
different sounds. So an alphabet shouldn't contain more than 35
letters.
The Phoenicians, who lived in Syria about 3000 years ago, were the
first to develop a modern alphabet. The Greek copied that alphabet
from the Phoenicians and the Romans copied it from the Greek and
improved it. This Roman alphabet is now used almost over the whole
world
The Roman Alphabet
The Greek Alphabet
The Hebrew Alphabet
Egyptian Hieroglyphics
An interesting link, which has many examples on different alphabets, is this one:
http://www.asklang.co.uk/publish/alphabet.html