Today's computer has been developed in many stages starting with the abacus. So the definition of the first computer is different based on what the questioner is thinking about.
1642 Pascal Mechanical add and subtract.
1670's Leibniz Mechanical add, subtract, divide and multiply.
1942 First electronic computer ENIAC using vacuum tubes.
1822 Babbage difference engine.
1890 Hollerith punch card Census Tabulator
1937 The first digital computer Atanasoff-Berry Computer - not programmable.
1949 First stored programed computer, EDSAC.
1956 First transistorized computer TX-O from MIT.
1960 first "mini' computer from DEC, please note, that mini in 1960 meant that the computer did not fill up an entire building or floor.
1973 IBM's the SCAMP, a demo prototype.
1975 IBM 5100 'Portable' Computer - it weighed 50 pounds and could cost up to $20,000.
1979 IBM 5520 sported a 130 megabyte hard drive.
1981 IBM 5150 Intel 8088, 4.77 MHz
1984 IBM 5170 had a 6 MHz Intel 80286 microprocessor.
WHEN and WHAT was YOUR first computer? How SLOW was it? How LITTLE memory it had? For me it must of been around 1984 because the 'speed' was a whopping 6Mhz. I was about 15 years old and my Dad worked for IBM.