What was the first thing man did on the computer?

Answer 1

Used it to solve arithmetic/cryptographic problems that were much too difficult to solve by hand methods before the results generated would be obsolete and thus useless.

1. Although it was not a computer, Herman Hollerith's punchcard equipment was invented to allow the 1890 census analysis to be completed before the 1900 census was to be performed. Hand counting methods used previously would have taken 12 to 15 years, making it impossible to meet the requirements in the US Constitution. Using Hollerith's punchcard equipment the task was completed in less than 3 years (faster than either the 1870 or 1880 censuses).
2. The Atanasoff-Berry Computer (1942) was used to solve systems of simultaneous equations far beyond the size that could reasonably be handled by hand methods. Unfortunately WW2 prevented realizing the full abilities of this machine as the university scrapped it shortly after its designer and builder left for war work.
3. The British Colossus code breaking computers (1944) were used to solve German High Command teletype messages so fast they immediately influenced Allied war planning to counter the German actions (it is sometimes said that Allied leader were able to read the German messages before the German leaders they were intended for could). Using the hand code breaking methods that Colossus automated, the German attacks would have been over many weeks before any of these messages could have been broken and read (which was what the Germans considered "good enough" to consider their codes to be "unbreakable").
4. ENIAC was first used to simulate Edward Teller's first hydrogen bomb design (the Super) and showed that it could not work, saving the cost of what would have been a failed nuclear weapon test just after the end of WW2. Later it was used for its original intended purpose of calculating artillery firing tables. It was fast enough to be able to compute each trajectory several times faster than actually firing the shell would take for the shell to leave the gun until it reached its target! Each table for a given artillery piece required calculation of many tens of thousands of trajectories covering all conceivable conditions.
5. etc.