Why play Starcraft 2? At some level, you play because it fun, but there are different kinds of fun. Some play just as in interesting diversion, and they don't want to think deeply or practice intensively, because they just want to relax. Some play just to socialize with friends. Others play with the passion of a competitive sport, and they derive pleasure from excelling at a difficult task. Understand why you want to play. To be a diamond-level player, you must have at least some commitment to mastering Starcraft through practice. 

For example, this is my understanding of why I want to play: I only have a couple hours each week to play Starcraft 2. I will try to be the best player I can be, given the limited practice I can afford. I am willing to do things that are not always "fun", such as practicing the same build 20 times, to become a better player.

Be humble. I think most new players, if they are honest, will admit to occasional delusions of grandeur. How they will discover the perfect build, or the perfect unit composition. How they will stun the world and their friends with their awesomeness and insight. Players who have played for awhile know better. There are a lot of smart people in the world, and Starcraft II is a complicated game. There is no perfect build, there are no perfect unit compositions, and that build orders and unit compositions depend on the map and timings anyways. If you want to be Picasso, you have to learn to paint first. Master the basics, learn to paint.

These are not in order of importance, they are just the basics.

Control Mechanics
APM (actions-per-minute, speed at playing the game) isn't everything, but you have to be able to translate the thoughts in your head into control movements in the game. 

Use hotkeys. Just use them. Even if it slows you down for the first 50 games you use them. There different hotkey setups you can use, like grid or standard, but just pick one and use it. 

Use shift-click. Shift-click to queue orders. For example, for your scouting worker, you can queue a string of waypoints to scout out multiple spawning locations. Once you have found your opponent, you can queue another string to scout around the base.

Use control groups. Create a systematic way to organize and control your units and buildings. For example, for Protoss, you can use groups 1-3 to control your army, group 4 for your nexus, 5 for gateways, 6-7 for robo/stargate, and 8-0 for upgrade buildings. This is a fairly popular scheme.

Macro 
Macro refers to building up your economy. This is the bread-and-butter of Starcraft II. Although Collosi counter hydra/roach, that doesn't matter if there are 30 hydra to your one Colossus.

Always build workers. There are exceptions to the rule, such as rush and rush countering, but don't use that knowledge as an excuse not to build workers. You should always be building workers, unless you have a clear and compelling reason not to. 

Spend your money. Try to keep minerals as low as possible. You need 400 to build a nexus/CC, there should be no reason to go above 500. Build army units. If you can't spend your money fast enough, build more unit-producing structures.

Don't queue units. Queuing units is not the same thing as spending money. The money disappears from the top right resource counter, but the money has not been spent productively. Let's be honest. Everyone queues units, especially workers. But the goal is to do it as little as possible.

Don't be supply-blocked. Anticipate your future population supply needs, and build overlords, pylons or supply depots as necessary.

There is much more to macro, but these are the basics.

What is a build order, and what is it designed to do?

You start the game with 50 minerals and 6 workers. From there, you can go many routes. You can decide that you want 50 drones before you make your spawning pool, you can decide that six workers is enough and go for a 6 pool, or you can do something in-between, that balances the need for economic expansion for the future, as well as military aggression now. 

Build orders are simply suggested directions to take. Remember that lots of smart people have played this game, and many different build orders have been tried. Over the thousands or millions of games that have been played, several dominant "themes" have emerged, and these build orders have been particularly successful. They are usually successful because they provide a good balance of solid macro play along with a "cute" timing, such as gateways #2, #3, and #4 finishing just as a warp gates are done, and the proxy pylon in the enemy mineral line. 

Build orders give you a general feel for an opening, but it is up to you to adapt. You may be hit with early aggression. The enemy may have turtled and spammed cannons. You still have to think, and apply the basic rules of good macro play.

This is too much to cover at once, but I just want to point out a few things. Only worry about this after you have mastered the basics. I feel that I am only marginally qualified to talk about these things, so I am being deliberately conservative. TL-veterans, feel free to correct me.

Maps. Maps can have a big effect on the effectiveness of a strategy. Look at rush distances, high-low ground interactions, placement of expansions, and lanes of advance. Look for places to abuse blink stalkers, tank drops, or muta harass and ling runbys.

Replays. Watch your own replays to see how you can improve, particularly macro. Check your mining saturation. Are you following the basic rules of good macro? Also get a good sense of the momentum of the game. Were you passive when you should have been aggressive?

Pro Replays. This is probably going to be the most controversial one. Watch pro-replays to understand their builds, but you may not want to copy them directly, because what works for them at 300 APM may not work for you at 50 APM. However, it is good to see what the goal is. Your goal, if you want to be a better player, is to become more like that.

Unit Compositions. Try to do more with less. You can't go up the whole tech tree, you can only go up one or two branches. Learn to use a few types of units to counter a wide range of different enemy builds. Even if what you have isn't the "perfect" counter, it is still vastly superior to sacrificing macro to get the "perfect" counter, only to get smashed by a larger army.

Be lean. See what you can get away with. Build fewer cannons, fewer spine crawlers. Can you fend off the reapers with just a queen and two roaches? Can you expand earlier? Can you get your upgrades earlier?

Liquipedia A SC2 wiki from the TL community
http://wiki.teamliquid.net/starcraft2/Main_Page 

How to Improve, by Ver This is about Brood War, but the concepts apply to Starcraft II. This may be too advanced for most beginners, but it is well-written and invaluable for understanding the mentality of high-level play.
http://www.teamliquid.net/forum/viewmessage.php?topic_id=135766

Day[9] Daily There are a lot of SC2 casters and commentators, and there isn't time to mention them all, and this is not a judgement on them. I will just say that Day[9] is very insightful and has helped my play. His commentary is very accessible to lower-level players. For those not familiar with the Brood War scene, understand that he has immense credibility. He knows what he is talking about.

http://day9tv.blip.tv/

Particularly useful episodes:
Day[9] Daily #132 - Back to the Basics: The Mental Checklist
Day[9] Daily #146 - Gold Level 2v2 Analysis
Day[9] Daily #163 - Gold Level: Fixing the Little Things 
Day[9] Daily #164 - Low Level Reps - In-game Decision Making