Part 2: Build OrderEarly Build Up Order for Base and Units:
General Rules/Theories for efficient build up in the early game can be applied fairly well to all species/races (Protoss, Terran, Zerg). These rules are generalized for a medium sized map, with medium defensive abilities. Adapt them accordingly to other types of maps. Only experience and testing will tell you which maps are best for what. Some maps call for more concentration on economy and/or techtree, while others call for more early fighting. I can give some concrete examples. Such maps that favor early offense are those that are small and/or open (don't contain only one narrow chokepoint leading to your base). Examples would include Orbital Solar Relay and to a slightly lesser extent The Hidden Shrine. Some maps are better to defend against early offense. Defensive maps would often include maps with a long path to your opponent, big maps, island maps, and maps that have one choke point leading to your base. Examples of defensive maps would be Dire Straits, Discovery, and to a slightly lesser degree The Lost Temple.Early Build Up:
When building up from the start, quickly separate the 4 peons/workers (i.e. SCV, Probe, Drone) as you put them on separate mineral crystals. Exclusively build peons till you are at 7-8 probes/7-8 SCV's/7 drones. Then build a farm equivalent (Overlord, Pylon, Depot). The general rule is to not build a building if it keeps you from making a peon, except when resources are far away from your townhall (Nexus, Command Center, or Hatchery) or if you are forced to make your farm (Supply Depot, Overlord, or Pylon) and barracks (fighting unit building) in a timely manner. Also, don't make a fighter if it keeps you from making a peon, unless they are attacking and you are outnumbered. In that case build men first.
The next rule to remember is to not build a building if it keeps you from making a fighter. For instance, if you want to build a ton of marines, don't build 2 barracks if you can barely afford to make a marine every time a marine is done building/producing in your first barracks. Instead, wait; when you have more peons/workers you will be able to afford to support unit production and build new structures at the same time. In the very early game never queue more than 2 units in any building because it is wasted resources.
NOTE: never get so preoccupied in early game with fighting or scouting that you forget your economy and build up order. Economy and build up order are the most important things in the early game.
Your first base should usually have enough men to counter an early rush. Usually besides Terran bunkers, no other defensive tower structures need to be built early on because it will just strain your resources. Don't even build the buildings that are used for making towers until later on in midgame. Building fighters is better in most all situations. Towers are best used for cloak detection, anti-air, and as a distraction for attackers - so use them as such.Map Types Affecting Build Order:
There are times when you want to break these early build up rules. It's impossible to say when they all are; however, if you understand economy and build up theories well, the times to break them will be more apparent.
Some good examples of when to break these rules include when you are on a "rush map" (a rush is often considered to be an early attack with early 'weak' attack units; a rush map is a map that is small and often open/not easily defended). You'll want to make attack units a much higher priority, so build your barracks earlier and make men instead of workers if you absolutely have to.
Another time to break these rules is when you are on a very defensible map. Sometimes on the defensive maps you'll want to go up the "tech tree" or expand quicker - hence you won't be building many fighting units. Instead you'll be building tech tree buildings or new halls (command center, nexus, hatchery) near away resources. In this sort of a game get your gas mine going quicker. Basically, break the build up rules when you need something out of the ordinary to do well in the early game.