(Disclaimer: this website isn't that good and was made for a coding camp)
Welcome to your resource for speedrunning! Speedrunning, if you're new, is the attempt to complete a segment of a game or the full game as quickly as possible. Speed Demos Archive is the official registry for records, glitches, and strategies. Games Done Quick is a charity orginization that runs two events per year (AGDQ and SGDQ) where speedrunners run around the clock to get people to donate directly to various charities. I personally run Mirror's edge, but you might choose something else.

Questions I would've liked to know the answer to:

What is "Any%"?	Any% is the category of speedrun that requires you to get to the end of the game with no specified percentage of missions or items or collectibles done and collected. there's also 100%, which requires the game to register you as completing it 100%. There are also game-specific categories, like SMW Any% No Cape.
How do I time my speedrun?	You can use the timers listed here or here and set up splits that mark important sections of the game. The time generally begins when you gain control of your character and ends when you lose control. (meaning your buttons/accelerometers/joysticks/keys/mouse omake your player do stuff)
What game should I run?	What game do you like? Most games have been spedrun at one time or another [1], so whatever you'll have fun doing. You might want to avoid games with absurd amounts of random generation, as that can skew times based on luck, not skill.