(after installing a game and map) To start a single-player game use one of the Lobbies for that. A lobby also allows to set more options and can download games and maps, too.
You can also start a game by starting the Spring engine ("Test Spring" in start menu or "spring" in command line):
Then select a game, map and AI and click "Test the Game"
Not all AIs will operate with every game!
See the dedicated page Playing on a lan.
Useful Hotkeys, Etc.
For the default keyboard layout please look at the Keyboard page.
The Spring Forum
Your question not answered here? Got problems?
Game Specific Questions
If you have a question or problem with a certain gameplay mechanic in a game, you can talk about it at the game's subforum
If Spring crashes/hangs, please look at the FAQ.