Everybody messes with the default settings. Here is a list of settings a lot of people change after the fresh install:

General
Dynamic Camera: it zooms the camera towards your cursor. Uncheck this! It's just dizzying to play with. There are guides on dyn cam toggling.
Controls: Some people change jump to W or even S instead of space.
Chat: Usually changed to enter/return (team chat could be ' or apostrophe)

Graphics
Borderless window: makes alt-tabbing much smoother (especially for those with multiple monitors)
V-sync: Uncheck this! This reduces your mouse/cursor response rate.

Apart from settings, you should make some changes in your server list menu!

Filters: Uncheck all of the filters. You might sometimes want to use "Has people playing" filter.
Ping: Arrange in lowest to highest order.

Steam
Right click on Teeworlds on the games list/library -> properties -> Set launch options -> Copy/paste "--silent" without quotes.