Note: this is a guide intended for bronze/silver players mostly, so you may disagree with some points here and there, but I believe its easier for new players to understand the game first before looking into deeper strategies/micros/etc..
If anyone thinks that there is anything this guide is missing, you are welcome to point it out and I'll gladly add it.
1.) The Basics
1.1) Probe Production
Perhaps the biggest problem I've noticed among lower leagued players is that they don’t make nearly enough workers. Generally speaking, you should never hold your probe production until you are around 80’ish probes. The key to a powerful army is a strong economy, which is powered by a lot of probes. Don’t worry about how making too many probes is going leave you without enough army, unless you’re going for some rush builds (k4g for example),there is never a time where lots of workers are gonna hurt you. You dont need to make worker non-stop past early game, but you need to remember to chrono boost out a pair of worker once in a while.
1.2) Chrono Boost
Chrono boost is another important thing that most low league players miss. You should never save up chrono boost off 1 base unless you are getting an important tech soon. Toss production and research are fairly slow so chrono boost is essential to keep yourself ahead in a game.
The importance level of things that needs to be chrono boosted are as follows (imo):
Warpgate tech (prob most important tech, worth saving up chrono for)>Extended Thermal Lance/Psi-Storm>Other Upgrades>Robo/Stargate Units>Probes>Warpgates (Late game when you have spare).
Of course you need to use early game chrono boosts wisely. Usually you should chrono out your 11th, 12th, 13th, and 14th probes. After that if you do not require a super fast warpgate you can maybe spend another 1-2 chrono on workers, but save at least one for warpgate. In mid-late game once your on 2-3base, you can feel free to chrono anything you need out fast. It will help a lot to get some vital upgrades out.
This is yet another common problem. Players tend to either make too many pylons thus wasting valuable resource early on or get supply blocked and have their production halted. Supply block at a bad time can easily cost you the game. It is quite hard to not get supply blocked at all during mid-late game. But I'd suggest that every time you goes through 1 production cycle of units (made some units from your production buildings) add 1 pylon for every base you have and maybe a extra one if you’re going supply costly unit like Colossi. Early game though, a good time to drop your pylon will be: 1st pylon at 9 supply, 2nd at 15-16, third at 22-24, 4th at around 30. This should allow you to stay ahead in supply early game without wasting much resource.
1.4) Production Buildings
1 Protoss base can support a maximum of 4 production buildings continuously making units. So you should not have more then 4 production building when you’re on 1 base. If you are going high tech units such as Colossi off of 1 base, you usually can only support continuous production off of 3 buildings (1 robo 2gates). More importantly, once you have expanded add more production buildings so that you can effectively turn your resources into army. You don’t need another 4 buildings up as soon as your expo is up but get them gradually as you saturate your expansion.