A large portion of the game revolves around your total number of colonists. The end goal is the maximum total production which is population multiplied by efficiency. Having a higher population means you can be less efficient and achieve the same result or to be equally as efficient as the other races and result in more production.
The best way to hurt the enemy's production is to "blow up" their worlds, or to destroy every single person on the planet. This reduces their population and directly reduces their ability to research and to produce ships.
Additionally, the game includes maintenance costs in its calculations. These costs are divided proportionally between worlds.
If you have two worlds each with a production of 100 and a maintenance burden of 50% you are getting the equivalent of 100 combined production to use for your own purposes. If you added a 3rd world with 100 production to the mix your maintenance burden would drop to 33% and your total production would increase to 200, doubling your output by increasing your production base by half.
Essentially speaking, each new colony is more productive than the last one, and this makes it very valuable to try to expand as fast as you possibly can while still protecting your planets adequately.
As far as gaining population, there are two ways to go about it. You can either (1) let it occur naturally through the base reproduction rate, or (2) utilize cloning in addition to #1.
It is often best to let the planet max itself out using the first method early in the game because this method is free and the energies they would spend on cloning could instead be used more efficiently to bring the world up to speed.
Each colonist can control a certain number of factories (think "The Jetsons") and these factories don't exist till you build them. If you clone a colonist and no factories exist for him to control then he is mostly pretty worthless.
Later in the game, though, if you have moved colonists away from a world (a strategically good idea sometimes) you will have factories sitting unused because there is nobody to operate them, this can be a very good time to utilize cloning in order to most quickly return to the maximum population where all factories would be utilized again.
Generally speaking, the efficient rule is to grow your factories first to the point where all the people control factories.
On Impossible difficulty, you start the game with 40 population and 30 factories. Each person begins play able to control 2 factories, so essentially 25 people are wasted. By focusing solely on building factories early you can "catch up" factories with population and get those 25 people working, as well as the people they create through natural reproduction.
On Simple difficulty, the easiest setting, you start out with 50 population and the same 30 factories so you have 35 people standing around doing nothing.
Getting these people working is priority number 1.
Population can create things through their own manual labor, but they are really bad at it.
Each population by itself can make between 0.5 BCs and 2 BCs depending on the civilization's technology level in Planetology (scaled evenly across the range from technology level 1 to 99).
Each population can control between 2 and 9 factories in the game, though, which each produce 1 BC. Most races are limited to 7 factories maximum per population, but a single race, the Meklars, can control 2 additional factories than they otherwise would be able to as their racial bonus.
That means the minimum production for 1 population (utilized most efficiently) is 2.5 BCs and the maximum is 9 BCs under normal conditions. At the outset, a single colonist is 5 times more effective with 2 factories under their control than with none (2.5/0.5=5x. At the end of the game this ratio varies depending on technology level, but with Improved Robotics Control VII technology researched, a normal race will be 4.5 times more effective with 7 factories under their control as compared to with none (9/2=4.5x).
You can also use factories to build other factories, increasing your production base at a very fast rate whereas if you just spend everything on cloning your production would mostly increase linearly (and at a very slow rate).
At the start of the game, it costs 10 production to build 1 factory and it costs 20 production to clone 1 colonist. You get 1 BC from factory minus 0.5 BC spent on cleaning pollution for net gain of 0.5 BC (as long as it is controlled by someone) and you gain 0.5 BC production from the 1 cloned colonist. Spending the same 20 BC you would get 1 BCs in production from factories as compared to 0.5 BCs production increase from cloning. Essentially, money spent building factories at the beginning of the game is 2 times more efficient than money spent on cloning.
Because of this, and because you always start with extra people that are not doing anything, the best strategy is always to put as much production as possible into building factories at the beginning of the game and to just pass turns until the number of factories becomes 2 times the population (the max they can control at the outset).
Once you hit that point, you can determine where you want to go from there.