The Abacus, around 1000 B.C. Of course, if you mean personal
computer, it started with the IBM mainframes in the 1950s, and
branched off for a while, finally creating the first personal
computer, the DATAPOINT 2200, in 1970, and the MITS Altair 8800, in
1975, then the Apple ][ in 1977, finally with the invention of the
IBM 5150 in 1981, the first modern PC. The IBM 5100, the first
commercially available portable computer, appeared in September
1975, and was based on the SCAMP prototype.
Sources: Wiki and Yahoo