The goal in Earth is to get the highest final NW or win a WAR. Here are the basic concepts to get you started, in order of importance:
1. You need a lot of land. Land provides growth for your country. However, you should also have enough military to defend your land. Otherwise you will just get landgrabbed and lose your land. There is no "optimal" defense. The best way is to look at the status of top 10 players and see how much military they have to get a general idea of how much defense you should have to defend yourself from other similar military-structured top 10ers. Obtaining land (also known as landgrabbing) is not an easy thing to do, refer to the "Landgrabbing" section for details.
2. You need tech for success. Tech provides (squeezes) the maximum income or production out of your land. As a beginner you should start getting tech during midgame (start of 2nd week in tourney for example). Tech can be obtained either from Research Labs or from the public market. If you're not a techer (see Basic Strats for details), you should either use your income (cash from military sales or simply cash production) to purchase tech on the market OR you can dedicate a "phase" in your game to convert (see Conversion for details) your buildings into all Research Labs and then research your tech from there.
3. You need construction speed to construct at a faster rate. Contruction sites provide faster construction speed as mentioned in "The Basics" section. Having a faster construction speed than others is an advantage. Every advantage counts towards your final success in Earth.
4. You need good defensive allies. Try to ally with the top 10 players in your game for maximum protection, if not then just find someone with a decent defense that can provide you with the protection needed. Defensive allies make up a good portion of your total defense so it is very important to find good allies. It is the best that you find def allies that are equal or at least around your networth because the maximum amount of military your allies can give you cannot double up your military total strength.
Land, Tech and CS level are the 3 things that determine how well you do. More land means faster growth rate; more tech (in specific areas favoring your strat) will allow faster growth as well as less maintence costs in some cases; more CS means faster build rate, which in return saves you more turns to do other things.
You must have tech in order to do well. The earlier u can get your tech up the better. Try to get your "critical tech" up (the tech categories that best relate to your main strat) before starting to tech any other categories for maximum growth.