Re: command
So, are you talking about the original C&C, or about Red Alert? The tech trees are different in the two games, and are different between the two sides in each game, too.
From what you say there, I'm assuming this is C&C1, GDI side.
If you click the Help button on the main menu, you will end op on the EA help page for the Remaster, which contains a link to a guide to get started. There, under "Gameplay basics", you can see the full tech trees for all four sides.
(There are some minor inaccuracies; Orca / Attack Heli don't really require the infantry production building, the Harvester requires the refinery, not the Barracks, and the guide kind of makes it seem like the Barracks produces the APC, while that's obviously done by the vehicle factory)
C&C1 does not contain medics; those are only available in Red Alert, to the Allies side.