Microsoft was founded in 1975 (then Micro-Soft) as a programming language company. Their flagship product was embedded ROM BASICs for various 6502/8080 CPUs. The company's first major product was a BASIC interpreter for the Altair 8800 microcomputer. They then started to expand into a compiled BASIC, and some limited games.
Their big break was with IBM and the IBM PC. While they were working on the embedded BASIC for the 8086 cpu, they learned that negotiations with DRI over CP/M were breaking down. They then took the lead to tell IBM that they had been working on a CP/M 'like' Operating system, and would sell it to IBM very cheaply on the condition that they could also sell MS-DOS to any other OEM that may be interested. IBM was still in its monopoly trial, so they figured this would make the platform 'open' and they wouldn't be harassed by any lawsuits over the IBM PC.
Microsoft famously found 86-DOS, bought the rights and had it ported to the IBM PC as MS-DOS.
Microsoft's first products were programming languages including:
The most famous of all would be Microsoft Windows.