This is a very broad topic, so I will try to break it down to the very basics, depending on your level of play.
Beginner (if you pick up a new game): Try to get a basic build order. Create building / gathering units constantly to push your economy. Learn the most often used Hotkeys and use them. Try to understand what your meaty and damage dealing units are and mass an army with them. From time to time make sure you upgrade your units. Push as soon as you have some big advantage (say good upgrade finishes).
Advanced (you understand the mechanics): Make sure you can execute a few build orders, that complement each other (e.g. rush, expand and tech). Try to use the best BO for the current map. Make sure you spend your money all the time. Push when you have an advantage. After you push get expansions to secure future income.
More Advanced (solid execution): Use the right BO, create units regularly etc. Use your fighting units to their potential. Use ther special abilities in fights and try to activate them again as soon as the cooldown wears off. Build good arcs, pull back injured units. But the most important thing is: Create new units while doing this. Try to act strategically: get expansions if you can defend them, prevent your opponent from doing the same if he overextends himself.
Professional (playing 4+ hours regularly): You have to consult forums / websites for your game and get better at execution and multitasking.
To be good you have to have at least a certain amount of Actions per Minute (>= 50). This does not mean you should just click random buttons. You need to have a certain speed to execute your strategies properly (synchronizing building and unit production, Micro Managmeent etc.). My point is: If you have less than 50 APM you cannot be good, otherwise you might be good. I state this because many RTS games support this measurement unit and for beginers it is a starting point to measure their performance.