First, I want to share a little background information regarding me and StarCraft 2. I started playing StarCraft 2 back when it was first released on July 27, 2010. After finishing the placement matches to determine my league, I was placed in silver league. However, after playing more games I was demoted to bronze league. Today, I still remain in bronze league but I feel that I have improved my game drastically since I started playing by executing five basic tips.
Macro over Micro
You always hear people say that you should macro before you micro. This is true to an extent. I never used to expand to my natural or any other expo. This was a big mistake. The moment I started to expand, the more games I started to win. Also, don't forget to constantly be producing SCVs/probes/drones. Once your main becomes saturated you should already have expanded and should be ready to transfer SCVs/probes/drones. Obviously, this doesn't apply if you are using a one-base build. Once you become a more skilled player, only then should you focus on micro. Also, make it a goal to keep minerals and gas below 500 each.
Constant Unit Production
A unit producing structure should always be training units. Don't waste precious resources on additional structures if they won't be constantly training units. Instead, you could be investing your resources into units with existing structures.
This doesn't seem obvious to many new players. It didn't to me. The key is map control and knowing what your opponent is doing.
Xel'Naga towers - If a map has Xel'Naga towers, then you should focus on controlling all of them. This can help reveal your opponents unit composition, or give you advanced warning if your opponent is getting ready to attack.
Look before you leap - If you're about to advance your units up a ramp of your opponent's main or expo, send one unit up the ramp to take a peek. This will let you know in advance if your units will stand a chance against your opponent's units. As as example, sacrificing a stimmed marine (50 minerals) might be worth it before you push all your marines into a line of colossi.
Scans - Being able to see and know what your opponent is doing without sacrificing units is powerful. If you deploy an orbital command, use your scans! I have been using the first 100 energy to call down two MULEs but using my subsequent 50 energy to scan my opponent's base. This allows you to quickly figure out what your opponent is doing in the early game that way you can counter accordingly.
Don't queue up units
This is a controversial topic. Queuing takes up precious resources that could be spent elsewhere. Try not to queue up more than two units. If you find yourself with over 500 minerals then you should probably be creating more unit-producing structures.
Don't get supply blocked
The most important thing to remember is never to get supply blocked. Always keep an eye on what your supply is at and plan ahead. If you notice that your supply is getting close to full, then build two supply depots to give you more leeway.
That's about it, for now. Keep in mind these tips are coming from a bronze league player. In no way are these tips perfect. They have helped me and I hope they will help you.