Building is an extremely important part of Tremulous. Both teams have a wide range of buildable items, many of which are essential in both attacking the enemy spawn points and preserving one's own.
This page covers the basics of building: how to place buildables rather than where to place them. Advice on constructing a successful and effective base is a subject best left to the more in-depth articles on strategy.
To place a buildable you use the "Primary Attack" button, normally the left mouse button. This will open a menu that lists all the available buildables, along with a brief description and the cost. Either select the buildable you want and click "OK" or double click on the buildable.
Once you have chosen a buildable, a translucent image of your choice should appear in front of you. You can move around until you get it where you want it, and then press fire again to place it. It will take a short time for the buildable to complete, and you won't be able to use or sell your construction kit until your build timer has expired, as shown by the small pie-chart in the bottom right of the hud.
If the translucent image is red, then you can't place it in the current location, so you will need to move it until it turns green. If you think you should be able to build it in the current location click fire and you will be provided with an error message which should explain why you have been prevented from doing so.
If you wish to remove a building you can use the deconstruct command, by default bound to 'e'. This removes the building you are directly looking at. To deconstruct a building you must have a Construction Kit or Granger and an empty build timer. Deconstructing a building partially fills your build timer.
When you have a Construction Kit or Granger all buildings will have a health bar over them, which will deplete as they are attacked. Alien buildings slowly regenerate over time but human buildings need some intervention from a Construction Kit. When you have a Construction Kit you can hold the Secondary Attack button (default is right click) to repair the building you are looking at. Like any other building activity, this is only possible when the build timer is empty.
All buildables, apart from spawn points, require either power (humans) or creep (aliens) to operate. This means that all buildings must be built within range of the reactor/overmind or a repeater/egg. When the power structure for a human buildable is destroyed or deconstructed, the buildable no longer operates, but when an alien buildable loses its source of creep, it dies.
Finally all buildings require enough physical room. All buildings have a square base, so therefore buildables like the armoury are wider than their models would suggest. The buildings take up exactly the same amount of room regardless of orientation, so there is no point rotating them.
Each team has a limited number of build points they can spend on buildables. For humans this is a limited amount of power provided by the reactor, and for aliens the overmind has limited sentience. When you are building then the remaining build points are shown at the bottom right of the HUD.
By default each team has 100 points to spend how they wish, although some servers change this value via the use of the cvar; G_humanBuildPoints.
The menus for building can be slow and cludgy, so many players use binds to speed up their building.